Thanks to both Harold & Jack for your input - and apologies also as I realise that I did not describe the problem quite as fully as I should (you heard what I said not what I meant to say - I should have re-read my posting earlier)
The issue is in fact purely with the automatic cycle. The heat works fine on manual (thankfully, at this time of year!), which eliminates the concerns about safety switch, fuel, power, etc. The only cycle that does not work is automatic heat. Manual heat, manual a/c and auto a/c all work OK. Iím baffled - and you guys are both right, I should call a technician and probably that is what I'll end up doing - but it's become a matter of principle now (man vs. machine and all that nonsense!) Thanks again for your input and if anyone has any further thoughts - now that I've described the problem more fully - I'd be very glad to hear them.