Just had a 'senior moment' and posted this as an answer...
My machine is now noisy. It souds to me like the pum is making the noise. When the heating comes on or the hot tap is used, there is a sound, which to me, sounds like worn bearings. Plus, when the heating goes off, before the noise would stop immediately. Now there is a sound like a motor slowing down gradually over three or four seconds. As if the bearings were before silent, and now you can hear them. Would that be a sensible diagnosis?
If it is, is changing the pump 'simply' a case of draining the system, changing the pump, then refilling?
I have found a supplier of Potterton parts and they quote a new pump for £56 plus the dreaded. It won't arrive before christmas if i order one now. How long would you think the pump will run as it is? I first noticed the noise last friday.
Thanks

Gratefull Rob in Bromsgrove, December 2008

I am not a plumber but your diagnosis sounds reasonable. If you can locate the pump you should be able to ascertain whether or not the noises are coming from it or elsewhere. Before you order a part from a supplier ask a local plumbers merchant because it may well be that 'standard' pump will fit and they will have one in stock. My pump has an isolator on either side of it so I should be able to shut them both off and then remove/replace the pump without draining. Maybe yours has too? Good luck

Richard, December 2008