As I suspected, the actuator is sticking.

The original is a Danfoss HSA 3, which is sticking.

I have a brand new spare which looks in all respects identical, a
Sunvic SD 2701. It was bought as a spare head for our system, before it
was all replaced a couple of years ago.

Can anyone confirm whether this is compatible please?

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk