We have a 4kW PV array and a ground source heat pump. Both were installed autumn 2010, and we're finding them to be a great combination.
We use the GSHP to provide domestic hot water and heating. To add to what chrismahon said, our pump has a periodic heat increase that takes the temperature high enough to kill Legionella, for normal use we find the standard settings are fine.
We have underfloor heating on our ground floor and radiators on our first floor. Insulation is absolutely the most important thing in this respect - we gutted our house (built 1887) back to the stone so had the opportunity to insulate to current standards, and since you're looking at new build that won't be an issue for you. We also have a wood burning stove in our lounge that we can light when there's a cold snap or we just want to enjoy it - I'd highly recommend this anyway as a backup. Since our entire house is electric only, the stove also gives us something to cook on should we have a long power cut.
Our Feed in Tariff payments more than cover our electricity bill, and that includes running our 12 acre holding and all the lighting, freezers, heat lamps and other paraphenalia that entails, and it looks like it will also cover the cost of power to our ancillary accommodation which is a one bedroom cottage that was built last year.
We were lucky enough to have the capital to pay for these installations, and to get the higher FiT, and to get an interest free loan for 2/3 of the PV, and to get a 25% grant for the GSHP. Our PV system cost £15k inc VAT, GSHP was £9k inc.
HTH, if you need any more info just ask.