I am not a plumber but you may find that a gas boiler does not like water entering it hotter than a certain temperature, or temperature difference, it may also ruin the condensing mode. Also there is safety. I is possible to quite easily boil water in a wood burner, that can cause problems elsewhere.
You may be able to do something with an inline heat exchanger and a temperature controlling valve though.

The hot water from the wood-burner will flow through the heat exhanger in the boiler, which will efficiently couple it to the air in the boiler and so extract some heat and send it up the flue.

I also expect it will be against regs but I'm no plumber either.

What David said +1.
However if you want to avoid a thermal store, which can be expensive items anyway, you could install the wood burner with its own rads as a totally separate system. If you can manage it a wood burner installed on a gravity system is generally self regulating in so far as you don't (can't) over cool the firebox by running the CH pump and taking out too much heat. (Which could happen if the wood stove was in series with the gas boiler, a point not yet mentioned!). If the temp of the fire drops then the flow stops, when the temp rises the flow starts and the hotter the fire the faster the flow. Of course you will need the extra wall space but a few carefully placed rads from the wood stove, when on, could down down the TRVs on the gas CH and save gas that way.

Thanks all for the input.

SteamyTea - good point on the boiler not liking the temp diff - I'll need to consider this.

Gyrogear - good point - all Rads have TRVs so the one's in the study would hopefully shut off when the log stove is on.

djh - thanks, hadn't thought of that.

Davidfreeborough - all good points thank you. The hot water circuit has 3 towel radiators on it without TRVs so as long as these were always open there would be a path for the water. Also would need to ensure there is a propr safety system on the log boiler for if the water got too hot.

Peter_in_Hungary - interesting thought. I'll consider it but possibly not practical as I wouldn't want to have to rely on only the log boiler for heat for any parts of the house.

Pile-o-Stone - unfortunately no chance of Solar Panels close to the place where I would put the TS but certainly an option over the other side of the house where the DHW tank is so was considering changing the DHW tank to take advantage of solar at some point.

Bit of a rushed post this but thanks again for all the comments and I'll continue to look into it. I might be better off going with an H2 panel or similar? Great to be able to get some input on this - i'll continue to post with my thinking .....

Posted By: MagsonDavePeter_in_Hungary - interesting thought. I'll consider it but possibly not practical as I wouldn't want to have to rely on only the log boiler for heat for any parts of the house.

Actually what I was suggesting is an additional CH run by the wood stove so the rooms / places chosen for wood stove heating would also have gas CH rads. Hence the need for additional wall space.

Posted By: Pile-o-StoneYou'll gain the advantage of DHW from your boiler stove and with a heat exchanger you can set the maximum temperature of the water at your taps so the piping hot water from solar and wood boiler doesn't scald anyone.

I would prefer to see a separate DHW tank because I think this gives better options. Your solar PV can heat the DHW and the CH TS can also heat the DHW if needed. Heat exchangers for DHW on a TS are usually plate heat exchangers and these can wreck the stratification (= loss of efficiency) in a TS due to the high water flows needed.

If you already have PV then unless you are already using it all it makes sense to divert to a TS.

Solar Thermal is pointless - its a one trick pony that needs massive storage to make any real difference.

If you go for a Newark TS you can get it with the coil in the tank and it doesn't upset the stratification as much if you draw off a small amount of water compared with a PHE system. Newark also allow you to add multiple Immersions etc and additional insulation as your losses will be quite high with a standard TS of any sort when you are storing the water at 75c +

If you want to keep the DHW tank in place then consider using a 500 litre buffer instead of the TS and use this to heat the DHW tank. In that instance adding immersions as dump loads would be split between the DHW (1) and the Buffer (2) to maximise hot water.

PeterW - you've hit on something that I've been worried about there, i.e. losses associated with a TS. I read in some places that losses are minimal but I've been concerned that there has to be some loss and if it's comparable to any benefits I gain, I'm simply wasting my time and money. I'll have a look at the Newark - thanks.

ringi - good input thanks and all taken care of. Over the last 6 years I've replaced all the single glazed windows with DG and draught-proofed all the doors and other cavities. There's 18" of insulation in the attic ..... and if we're a bit cold we put a jumper on before we turn up the heating. The gas boiler is v old but renewing that is part of the new installation plan.

I didn't regard having a back boiler on the WB as a pain really, it was more of a case that we are definitely going to have a WB in that room, as much for aesthetics as for efficiency - so I thought why not include a back boiler and divert some of the heat to the CH rather than just heating up the one room.

<blockquote><cite>Posted By: gyrogear</cite>Well, not to spoil the party or anything, surely a simpler route would be to install the WB, then use a heat-recovery system on the flue, to borrow heat that can be ducted out to neighbouring rooms...

These things cost around 150 Euros in France... (but ducting will cost you a lot more )
http://www.bricodepot.fr/st-brieuc/search/node/;jsessionid=6D1B26B12FE99208894661DF8A93CFE0.node41kit%20de%20distribution%20d%E2%80%99air%20chaud?query=kit+de+distribution+d%E2%80%99air+chaud&pageSize=20&categoryId=navCatalogRootCategory&sort=priceAsc

gg</blockquote>

I really like this idea of robbing heat from a flue for another room, I can't convert this to English but get the gist, are there any English retailers that I can get into from?

Posted By: Peter_in_Hungaryany chance they would over cool the flue, especially if the stove was being run shut down.

Yes, I suppose that there is always that risk...

I should imagine that the heat recovery unit is delivered with technical instructions to similar effect, viz. don't open it up until the stove is at operating regime etc.

In that case, the risk should not be any greater than abusing any other mechanical equipment, such as, say, over--revving one's car on a cold morning... .

I don't have one of these units anyhow (no space), but what is apparent is, *ANY* stove will (by definition...) have its flue cooling, as it is run down !

Isuspect that the normal mode, is to give the stove a good blast at least once per session, to get the s**t out of the flue.

Personally, I bought a WB that was intentionally at the low-end re my heating requirement, so that it would need to be run HOT and fast (which is when they give maximum efficiency...). Also, after many months, I bought a flue temperature device that indicates when the flue is at optimal / sub-optimal / dangerous level; a good investment: on first using it, it was immediately obvious to me that I had been running the stove *way* too cool...

gg