Apologies for probably not using the right technical terms...I'm not a
plumber.

However, I appear to be having issues with the motorised valve thingy on
my heating system. Before calling out a plumber, I'd like to know more
about it, and likely causes of problems.
The problem is that it switches from heating to hot water OK, but no
matter what I try, it will not switch from HW to heating automatically -
I have to use the little lever on it to move it back.

Is this a wiring issue, or just a knackered valve ?

TIA,

Ian