I installed a high output ASHP connected to the existing radiator CH, but retained oil as back up. I also installed an array of Solar voltaic panels.
The ASHP is very efficient with temperatures down to -2C, then it needs the oil back-up to boost the temp in the house.
The combination of ASHP + SV reduced my annual energy costs from £4000 in 2010 to £350 in 2012. However the winter of 2012/13 will have seen my energy costs rise to around £750 -- still a massive saving for such a large house
I have been interested in "reducing" waste for over 10 years and in previous houses had made them more efficient + used solar thermal to heat an outdoor pool when they said it could not be done.
On moving to a 1920's house (3200 sqft) with oil heating I had a project to make the house as zero energy input as practical possible