I'm not sure what make the valve is - I can't see any identifying marks
on the body of the value, but access is rather cramped. Also, someone
(not me) has painted it. The metal seems like plain brass (no zinc
finish).

The motor is a synchron motor. About 1 1/2" high. Round on the top and
tear-drop shaped on the base with a small toothed wheel emerging
slightly off-centre.
It looks like this:
http://www.screwfix.com/sfd/i/cat/84/p1055284_l.jpg. It sits under a
metal cover above the valve.

Are you saying that when the valve opens fully it trips the microswitch
which turns on the pump and boiler? If so, would it make sense to
disconnect the microswitch from the junction box and use a continuity
tester to see if trips when I turn on the hot water? Or am I still not
understanding?

Andy