potterton puma 80e?

My machine is now noisy. It sounds to me like the pump 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 (normal)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 prognosis?
If it is, is changing the pump 'simply' a case of draining the system, changing the pump, then refilling plus bleeding to remove any air?
I have found a supplier of Potterton parts and they quote a new pump for £56 plus vat. 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. I have been building and servicing air conditioners for several years, including split systems with water cooled condensers, so i am familiar with pumps.
Thanks

Gratefull Rob in Bromsgrove.
December 2008