The pump on my central heating system has failed - just after the boiler
was serviced the boiler started to make 'cranking' noises as if it was
too hot, so I turned the pump up to top speed (it has been on the middle
setting for over 10 years) but a few days later there was no circulation
(pipes going into and out of the pump were cool, no heat in downstairs
rads) even though I could feel the pump vibrating.

It is an SMC Comet 2-130. I understand SMC don't make pumps anymore. Is
it going to be a problem getting a replacement pump? There seem to be
isolating valves above and below the pump, so presumably no need to
drain the system (it's an ?indirect? system - ie where an air bubble in
the hot water tank divides the central heating water from the hot water
supply)? Any ideas how much I'll have to pay for having the pump
replaced?

Thanks,
John