Designed properly ASHP's work really well in well insulated buildings and with the Governments support, the extra capital install cost is easily recovered.
Sparkgap's post is precisely why you should use an experienced, qualified professional to design and install - same as you would for an oil or gas boiler. What he points out we could have told him before he even started, classic carp designers..
Heatpumps work best with lower flow temperatures. As an example we have just completed an install in a refurbished 250m2 property - original heat load 38kW, new heat load, after refurb 9.8 kW, downstairs ufh, 1st and 2nd floor radiators 4.3 x bigger than oil however due to the much lower heat load, they still aren't large. (designed flow temp 40°).
A heat pump will be cheaper than mains gas, lpg or leccy to run, however with oil at a it's lowest price for 8 years, oil is currently the cheapest form of heating you can install.
Not for long, with the BRIC group massively increasing its energy consumption then the global price will go up as quickly as its come down.
SO, if you have a short term outlook go oil, (5 years)
If your looking to live in the property long term 10-15 years or more, I'd go for heat pump.
On the basis that insulation is ALWAYS cheaper than heat, there is no reason at all why he can't exceed current building regulations insulation levels (loads of our customers do
he does need to use the right techniques though if he needs to maintain the breathability of the walls.
More and more consumers are becoming ware of the benefits of a good heat pump installation - ask any of our clients.
Done properly a quality renovation to current insulation levels will always sell more easily and for a higher value than a not so well insulated oil fired property.
So if he's looking for the cheapest conversion, skip on the insulation, shove an oil boiler in and flog the property quick.
If he's looking for a property that he's still enjoying in 15 years, insulate it well and put in a heat pump, over a 10 year period, both will work out the same cost.