I think the control unit that you have in mind is a bit expensive

You could have your own bespoke one made or make one yourself all the parts are available on the high street. A simple controller that switches power so that you use it rather than exporting it

I would think that you could use all that you generate plus some so may be connect up direct to the water tanks, not so sure about an immersion in a tank of wax, what happens when it fails?

Thanks Tony, I guess if the wax hardens it gets stuck! My nightmare is always a boiler breakdown we have had it a couple of times over the years and if it all goes really hard (actually takes a few days to cool fully) it takes at least 2 days to remelt. Luckily I live next door so I very sadly can check pretty often (weekends I turn the boiler down to just keep the wax just liquid and no more). I guess the thing I don't want to do is import electricity from the grid and my knowledge is zero on how you can set this up so the system knows when to turn on and off, is this something the sparks we use would know?

There are a number of topics over on the Navitron forum discussing lower cost alternatives to the EMMA. Maybe a search there would help you?

why not send all the power to an immersion in the water system?

Could have a number of heaters, coming on sequentially.

First thing you need to know is the energy input into your water jackets and the volume of water, this will be lower than the output from your boiler (is it gas or oil?). Input and output temperatures would be useful too.

Then look at thermally insulating them more. Wax can store a lot of thermal energy, so you should be able to reduce the input some more.

Once you know that you are then in a position to start thinking about heating electrically. The way I understand it is that the voltage from your inverter is slightly higher that the local grid, this causes 'the faster electrons' to reach the nearest load, therefore using your power rather than drawing from the grid (I have never yet found out why it does this and the water analogy for electricity does not work for me here). So as long as you can generate more than the first of several heaters, you will be using your power.
The 'box of tricks' to do this is probably a current/voltage sensor and a load of contactors. Soft Start electronics may work to lessen the start up load (not really my field but we used them on a very large sauna installation and it saved tripping out the whole leisure centre).

As for solar, I think this is probably a non starter as you need to consistently have the water at over 75°C, and probably quite a lot of water as well. Cornwall has a mean solar radiation of about 135 W.m^-2, and really peaks over 1000 W.m^-2. So for every square meter of effective solar collection you would get somewhere around half that. a 70W heater is about what is used to keep a 1m fishtank at 24°C.

Work on the numbers and losses first. Knowing where you are and then using less is the way forward.

Instead of putting numerous immersions into numerous water jackets, why not just put say three immersions into a single well insulated hot water storage tank i.e. accumulator. This tank can be charged from various sources incl. your boiler. Then you are only left with the hot water distribution to you insulated water jackets, most of the plumbing etc. most likely already exists. So simply tapping into the exixting system and then a matter of pumps valves and thermostats.

I did suggest earlier using a cheap electric boiler in series with the oil fired one you have already. That would be simple to plumb in and would boost the water temperature up when the turbine would normally of been exporting, reducing the firing time for the oil boiler.

Posted By: tony...........................downside of an accumulator is that it is another place to loose heat from.

Do you loose heat from your house tony? You tell us not much, therefore why should a well insulated accumulator loose much too.