"Aidan" wrote in message
oups.com...

Your system has (or had) a corrosion problem which is corroding the
insides of the radiators. Since it is a sealed system, the usual causes
don't apply. It may be an old problem if the system once had a feed &
expansion tank with an older boiler. If so, it should have been flushed
before fitting a new boiler. You need to find and rectify that fault
first.

I agree, the previous homeowners should have had the system flushed, but
hey. BG engineer said it could be old flux etc. from when they had the new
radiator valves fitted, that had been left in the system to corrode.

The radiators are quite modern, about 10 years old, and don't have many cold
bits at the bottom, just 1 bit that isn't as scorching as the rest, so I'm
hoping the rads themselves are okay.

Thanks.