They have a Horstmann C27 (code 11225) at £26.49 and a Danfoss FP715
(63760) at £35.99.

Anyone got any views on which might be better? It would be used in
conjunction with a motorised valve on the DHW and an Eberle programmable
room thermostat which were installed some time ago.

Assuming you have a modern Part L insulated cylinder, replace it with this:

http://www.screwfix.com/app/sfd/cat/...84701&id=50424

At 1.65, it is much cheaper than the Danfoss.

Christian.