On Mon, 11 Oct 2010 12:00:27 +0100, Roger Mills
wrote:

If the actuator motor or micro-switch fails, you can just replace the
faulty component - or the whole actuator if you prefer. Neither requires
draining the system.

If you're really intent on replacing the wet bit frequently, stick a
full-bore ball valve either side of it so that you can isolate it
without draining the system!

Sorry, I think you misunderstood. I was only planning on draining and
replacing the valves once: to change to a more reliable brand!

When the Horstmann actuators failed, I didn't realise I could get
inside them and replace the microswitches but Screwfix sent me
complete replacements anyway, so I took the actuators off the valves
they sent, and like you said, just swapped the dry halves over.