My Glow-worm Hybrid ASHP was installed in Sept. 2012 by Hybrid Heating Ltd and the installation was carried out superbly and efficiently. It has worked satisfactorily after initial teething problems, mainly due to the different way to use the heating compared to the existing oil heating. The rads. were upgraded to take account of the lower water temp. provided by the heat pump and has been the sole source of heating during the winter. I still have issues with the fact that the outside sensor seems to have priority over the room sensor to control the H/P, but by placing the room sensor in various places the problem is largely overcome. It is too early to assess the relative costs yet but early indications are that there will be quite a saving over oil.
Get the rads. upgraded when the initial installation is carried out to avoid having to do it later if temps. are not high enough.
Get a reliable firm to do the work, ( Hybrid Heating did a good job for me !!! )
Feel free to email me if I can be of any help.