In reply to Pedro50:
The heating engineer sitting next to me as we drink our after-work beer says;-
Pro's, good retrofit for properties with normal rads, if one system goes down the other keeps going, "normal" hot water supply and you can play the energy price changes.
Cons, higher initial cost, the interlinking is more complex and the service costs are higher.
They are sold by all the major manufacturers here in Germany but my buddy says he's never installed one in a new-build, only as retrofit where there was an existing gas/oil system
I've pondered on fitting a ground source pump into my existing pellet/wood system but the price of electricity puts me off.