Recently moved house and found we have a very noisy/clanking boiler.
When the boiler has been on for about 20 minutes it starts for a few
minutes then cuts out and starts up again. This is a typical example
when the boiler is on. Air in the system you may say, but I have bled
the radiators about 30 times with little or no air coming out now. We
have been here 2 months and drained the system twice because of a
previous problem and a radiator relocation.

Any ideas?