Not sure whether my system is Y or S plan but based on what the
engineer said I would think it is Y plan.

The motor I mentioned is indeed the motorised valve motor.

I have a Potterton EP2002 control panel.

Really ought to know what make my boiler is but I can't remember.

Hopefully there is an engineer at home working on the problem as we
speak, with some knowledge of the electrical side of central heating.

The engineer who came round on Friday said the on-call guy for the
weekend wasn't qualified to work on gas systems (not Corgi registered)
which I thought was particularly useful, not.

Thanks for your prompt response,

John