Thanks for the ideas gnole,
I just wanted to give some feedback on my water heater so far. I already had an unused 6mm cooker cable in the loft not far from the pump installation circuit, so this reduced the cost and complexity of the installation.
Disclaimer: Any technical details here are just my opinion, and you should do your own research and take responsibity for your own actions.
To understand what I am about to explain it is important to understand that the greater the power of the heater, the faster the water can flow at a given temperature. A kettle is usually about 2KW if it was 7KW it could heat the water 3.5 times faster. Because the power input is more limited than typical household combi boiler, this means the rate of flow cannot be as high as from a boiler to achieve a specific temperature. A lot of the more traditional water heaters on the wall over a sink are about as powerful as a kettle so the flow rate is slow, these heaters are however on a high powered circuit and can provide water at the same rate as a shower, which would not be good for bath but great for just about everything else.
I made a few "mistakes" which I will relate here so you can benefit from my experience, firstly I made a mistake over the rating of the heater. I had assumed that the limitation on the loading of the 6mm for a water heater would be the same as for a shower. Showers over 7.5KW, are required to utilise 10mm cable, so bought a 7.3KW heater. (I can't see any technical reasons why they are different but I assume it is just greater level of safety margin being require for showers.)
However, upon reading the instructions, they stated I could have run upto 11KW heater on my 6mm cable. So this means I could have heated the water faster on the same cable. This is not a disaster but I could have filled my kitchen sink 30% faster!
The other problem I am seeing is the flow rate seems to be restricted, more restricted than it needs to be, This model came with an optional series control/stop valve. It would be fine under a basin but for my kitchen sink a little slow. I should like more control and I think I am going to replace the contol valve with one of own.
If I seem negative about it I am not, I am really happy with it, I just think I could have done it a little better and I plan to make improvements to the flow rate. During the winter I still plan to use my boiler to raise the hot water temperature but have reduced the time the water is heated significantly. I expect it to make a noticeable saving on oil and only a small increase in my electricity bill. Time will tell.