Hi guys, I have a problem which I believe to be the pump, but I thought I'd ask people with more experience before I go and replace it. The radiators downstairs are cold and upstairs hot. I had a guy in doing a service on the baxi back boiler and he said the pump was fine, that it was probably sludge and wanted £450 to fix it. I had a look at the pump and its starting to spin up, then cuts out. The flashing lights give an error code in the manual which just says unstable operation. I noticed while the boilers running, theres bubbling in the hot water tank. What else can I check to diagnose or fix it. The back boilers was just serviced and is working fine.