Insulation should be the first concern IMHO

Back to basics first.
You need to know how much hot water you use, and how much your heating load is. That is a case of measurement.
Then you need to start looking at the best way of distributing that energy, DHW and space heating need to be considered seperately, they do different things, at different temperatures and at different times i.e. DHW is about 40-50°C delivered for a short period of time, but a large volume and then discarded. Space heating is, depending on delivery, some where between 35-70°C i.e. UFH or conventional radiators, and then the transfer fluid is recirculated via a heat exchanger to be reheated.

All along the way you have to think of system losses. You ask if 10% is acceptable. Hard to tell, it sounds low, but that depends on how it is measured, at what temperature, what time periods. You can, for example, just look at DHW and fill a vessel with hot water, and then pour it out immediately, the losses will be very low, or you can heat one up to the maximum temperature it will stand, not use any DWH for 23 hours and then use all the hot water. You will get very different results.

As you have a solar thermal system fitted, you ideally need to start the day with a cold store, let it heat up during the day, and when it gets to maximum temperature, use it all, then let it start to reheat. Rather weather/season dependant and the maximum achievable temperature will vary, as well as the losses, but that is the nature of ST. Why it is generally cheaper and easier to use photovoltaics, but that is for another essay.

Now space heating is generally at a lower energy delivery rate, or wattage (you can work that out from your radiator area and temperature differences) even if the total energy delivered is greater than for DHW. Energy (kWh or joules) is not the same as power (watts or joules per second), try not to get them confused and use them in the appropriate places, though we all often make mistakes with them. So you have a heat source (the smoky and smelly, labour intensive, expensive to run wood burner, a subject for another essay, or 1000+ post on here) that can deliver energy relatively quickly 7 kW to water. So how fast can that heat the water for bathing? to keep the numbers simple, I shall assume each person need 100 lt of water at 40°C delivered. The incoming cold mains water will be somewhere between 4°C and 18°C over the year.
1 kg of water takes 0.00117 kWh of energy per degree kelvin increase in temperature. So 100 kg (100 litres) raised from 4°C to 40°C takes 4.2 kWh, from 18° to 40°C it takes 2.6 kWh. The stove delivers at 7 kW.
kWh / kW = h (or time).
So 4.2 [kWh] / 7 [kW] = 0.6 h (or about 40 minutes), 2.6 [kWh] / 7 [kW] = 0.4 h (or 25 minutes). So somewhere around half an hour burn time per person per day on average (not strictly true as there is an initial warm up and final cool down times from the stove and you may have differing residual temperatures in the store).
So for two people, you could just get away with a 200 lt thermal store that is at a mean temperature of 40°C. But it is generally kept higher than this for many reasons, mainly the fear of health issues and the fact that a wood burner delivers at a higher temperature. So your store may well be at mean temperature of 80°C initially, or enough for 4 people. But that will increase the burn time by two.

Now let assume that you need somewhere between 5 and 50 kWh/day for space heating. Your heat source, if it is not contributing to DHW will deliver at total of 11.5 kW (7 kW to water, 4.5 kW to room).
So 5 [kWh] / 11.5 kW = 0.43 h or 26 minutes, at 50 kWh heat load, 4.3 h or 260 minutes.

So how big does a thermal store need to be to deliver the the best, median and worse case scenarios.
If we assume that there is a 20°C temp difference between the flow and return temperatures, then:
5 [kWh] / (0.00117 [kWh] x 20 [°C]) = 213 lt, 50 [kWh] / (0.00117 [kWh] x 20 [°C]) = 2,136 lt
The median heat load will be 22.5 [kWh] / (0.00117 [kWh] x 20 [°C]) = 962 lt

You need to then add on the sizes for DHW, so another 200 lt, and then account for the losses, say another 20% in total.
So somewhere between 500 lt and 2,800 lt, with a median of 1,394 lt.

That is not the complete story though, as you can, on the coldest days keep running the heat source for longer, in effect replacing the house losses as you go. This will be in direct proportion, so double the burn time, halve the store size.
So taking the worse case scenario of 50 kWh/day and doubling your burn time from 4.3 h to 8.6 h will reduce the store size to 1,521 lt, your median will be 680 lt.

The smallest heat load is really irrelevant as if you just needed 5 kW/day, you would just use a fan heater, or accept you are a bit chilly in the morning.

There are other alternatives, using direct electrical heating for the DHW, this can be an modulating inline heater that can take advantage of any residual heat in the store from the ST system (what I would do), or use a combination boiler that just heats what you need, when you need it.

Really comes down to knowing what you need and when (so do some measuring now), how much room you have, and how much you want to spend.

Posted By: gyrogearGood Post
+1

+1 and if we are mentioning stickys or thinking about piller threads or core posts or what have you this is the sort of thing people need to see first, before they even know enough to know they need to ask a question.

Here is one, no experience of it:
http://www.gotankless.com/stiebel-eltron-DHC-E-series-tankless.html

You can always hunt down our old mate JSHarris over at buildhub.org.uk, he has researched them a fair bit and has one fitted.

The things to make sure is that it can accept hot water as the input, and that it is big enough for your needs i.e what may be large enough when the incoming water is at 40°C may not be when the incoming is at 20°C.
A second thing to remember is that they are really there to just lift the preheated water up by a few degrees, they are not, in this configuration, there to be the main heater, so knowing your desired flow rate is helpful.

+1 for the above. Excellent pieces of kit, well made, plug and play, straight out of the box.

Posted By: Fred56My heatstore is going to be E7 with a solar coil in case I can ever justify the investment.

Right, ditch the idea of solar thermal and spend the same amount of money (plus what you save on the extra coil) and get some PV fitted. Even a small 1.5 or 2 kWp system is almost certainly going to be better. It can also contribute to the running of the inline heater during the day.

As an alternative to the Rinnai option, Intergas boilers (which are technically a combi), can be set up as a water heater alone, because unlike many combis they do not have to dump excess heat from the HEX back into the heating circs after a hot water delivery, they have no plate either which again requires connections to heating circs.

The main block contains heating pipework and a hot water coil, the parameters can be changed to set the boiler up as a combi/system boiler or water heater to suit.

Their Rapid variant is available at around £565 incl flue and jig.

I was going to fit the 11e myself (illegally obviously) but it is the external variant and I already fitted 2 gas hobs and 2 gas ovens myself. Over here the 11e is eqiv of £460. None of this helpful I realise! Unless you have done some plumbing before and get the external version......

If we had e mail alerts, the OP would be back!

Posted By: SteamyTeaSo why we talking about gas boilers?

25kg bottles of gas can provide an awful lot of HW by taking

Posted By: SteamyTeaadvantage of any residual heat in the store from the ST system

Steamy. No don't have gas.
There is no mains gas in this bit of Cornwall and it is not possible to install an LPG tank in the garden due to the proximity regs and some BT services running through.

The water lag thing bugs me and makes the kitchen lady grumpy. I reckon we would waste about £25 per year on the water bill just due to waiting for hot water at the kitchen tap. Add the waste energy in the hot water trapped when the tap is turned off and it gets worse. I know folks say it's not waste but it is. The pipes are under a suspended floor and it is very well ventilated down there.

Steamy.
I had not thought of the tap. Will look at it.
I've done secondary circulation loop before on a barn with a very large footprint. It works but it costs in pipe, bronze pump and heat loss. It destrats the cylinder though.
Heat loss is the thing. I never measured it because I had no means of doing do. I have seen a few district heating systems and the losses are big. Setting aside the mud geysers just insulated pipes lose more than I thought. Last winter I rented a cottage on farm holiday site. It had a biomass district heating system with each unit having heat metering. The owner monitors performance and finds 35% of all heat sent out is lost in the distribution piping. Interesting.

Yes, look into the tap. I've read good reports about the higher quality ones, see the aforementioned JSHarris once more on his buildhub blog.

Don't bother with recirculating, imo.