Thanks to the advice given here, I persevered and got the actuator off.
I tried to turn the spindle on the brass valve, but it won't turn it
with my fingers, or even with medium pressure with pliers.

I take it that this means the brass bit needs replacing. That is not
good news - I have never drained down a system and I don't even know
where the drain cock is for this one.

I had found a replacement actuator on the net, but I'm not sure now if I
need one - when I run the heating with the actuator off, it clicks
throgh 90 degrees, switches off, then a few seconds later does the same
again. Does that mean it's OK?

Even if it is OK, I haven't been able to find the right brass bit to go
with the existing actuator so presumably that means I'll have to get a
complete new assembly.

This raises so many questions for me - must it be honeywell like the
junction box it connects to?, must it be a particular type?, what size
does it need to be?, will the wiring requirements of a different valve
exactly match those of the old one?, where is the draincock?, do I have
to drain the whole system?, etc. etc.

This may be blasphemy to this group, but I'm beginning to think I might
be better getting a professional in to tackle this. What do you think?

Even if I do sell out and call in a pro, I'd like to thank everyone for
the excellent advice and support - it's improved my understanding of how
all that stuff works no end,

Steve