We couldn't mount a panel on the roof because most of our roof is non weight bearing, it forms the passive solar heater for keeping the house warm in winter, so had to site the water heater on the ground and MMC's free standing unit works well then. As you mention flat panel units are don't seem that much cheaper these days and when we contacted an installer the price including installation was totally unrealistic, whereas the MMC installer was happy to cut costs and have us do the manual labouring and he brought his mum along to look after our hatake

I'm thinking of putting another unit on the cafe (which can go on the roof) and currently looking at the options, is your flat panel unit pressurised and refilled by the domestic cold water system as you draw off hot water ? I'd need that to be able to hook it up to the water intake of the gas boiler.
As you and Ian have said the one drawback is how it empties most of the tank when it gets too hot, we've only had to let off excess heat two or three times this year but I'd prefer it if we didn't have to ever. I'm sure there is a solution for this and am currently looking into it.
I couldn't agree with you more Brett that given the savings made with any type of solar heater I'm really surprised that not every house in Japan is using them. Would seem to me to be a win/win situation for the householder to me.
e2a:
Sorry forgot to say. Ian when fitting the relief valve was it a temperature and pressure relief valve or purely a pressure relief valve ? I'm wondering if fitting a more expensive American or European one would solve the problem.