I've fitted about 5 Amptec electric flow boilers. Various reasons (no gas in properties, and no garden for oil tanks, and various other reasons).
They've been great.
There's a dirty great big cable on anything over 9KW. 10mm T&E is a swine to run. However I run it, and get the sparky to do his bit connecting it to the consumer unit. I have enough knowledge to check to see if its viable (a spare MCB, an RCB, a big enough main fuse/supply ect), however if you don't know this sort of thing, get an electrician to check it will work before you order the boiler.
Finally I should add, that we've just bought a house off the gas grid, and I've taken out the old warm air heating and fitted new wet central heating, which is warmed by an 11KW Amptec boiler. So they can't be that bad. Just ensure you get the right electrical tarrif. There is economy 7,10,11 and various others.