Hi , perhaps a bit late for the work you are doing but i thought i could perhaps help.
As other parties have correctly identified the most appropriate buildings for heat recovery systems are those that are air tight. Others have also mentioned that they want to retrofit internal insulation. These two topics are linked quite closely.
The risk of internal insulation in older buildings is one of damp endangering the fabric of the structure but also the health of the occupants through mould spores etc. (ie sick-building syndrome)

A well designed mechanical ventilation and heat recovery (MVHR) should allow the safe installation of modern cost effective insulation materials.
Only once you have done this should you consider the heat pump. Heat pumps have a lower flow temperature so can only be used with heat delivery systems that designed for that purpose ie underfloor heating or low surface temperature radiators. Heat pumps are fantastically efficient in efficient buildings, in inefficient buildings there is a very real risk of increasing energy consumption and costs.
Depending on how far you take all of this the next obvious addition would be electricity generation through PV, wind, or hydro.
I know it starts to look like a very serious shopping list. What you could end up with is a financially and environmentally sustainable home that not only protects you from rising energy costs but also provides an income through the Feed In Tariff or Renewable Heat Incentive (RHI) when it starts for domestic properties in 2012. Better still, team up with a neighbour to jointly heat 2 or more properties to access the lucrative commercial RHI.
