Hi Lindzy, sorry can't help with Peterborough as I'm in Cheshire, and the only other I know is in Birmingham AKA Motormech that Nello uses.
Or maybe a bit nearer is Autolusso in Dunstable?
http://www.autolusso.co.uk/services.html
They are highly regarded in the Alfa world and are roughly an hour from you.
As Andrew says as the water pump is driven by the belt it only makes sense to change it at the same time as they already have the car stripped down. It's only and extra two bolts to undo and the cost of the pump is fairly cheap.
If there's 84k miles on the pump and you have no evidence that it has been changed in the cars lifetime, I would take the opportunity to do it now, as it would be at least 5 years old now ???? The reason that most garages recommend a pump change is because most are ran by the timing belt now and most timing belts are now in kit form and include new guide bearings, belt stretcher and some include the pump and gasket, when the new belt is fitted the new stretcher puts a different tension on the belt (because it is new) and this may be more than the old belt, which is a different push or pull on the old water pump (if it hasn't been replaced) what often happens is, that 3 or 4 months after you replace the timing belt and don't renew the pump, the pump begins to leak because of the different tension
Apologies for the rambling response, your eyes have probably glazed over
