Hi,

Are you sure that the knocking is related to the heating ? Cos I've
had intermittent knocking which was specific to a single radiator and
was easily fixed by replacing a new but presumably defective TRV, and
I've also had extremely loud knocking which was caused by the toilet
cistern being fed directly from the cold water main (rather than from
a tank). The water was quite high pressure and used to jet out of the
valve quite hard, so much so that the float used to bounce as a
result. This would set up an oscillation, whereby the float would
bounce, let in more water, which would cause the float to bounce, etc
etc etc. Doesn't sound like much, except the shock waves ran right
through the cold water piping and the noise was terrible. It took me a
long time to realise that flushing the toilet was the cause, as it
would happen when the cistern had almost refilled, i.e. when it
'seemed' no water was being drawn. Drawing cold water off at a tap
when the banging started settled it down temporarily, and a permanent
fix/bodge was made by suspending a damping device from the float arm
(cable tie and a plastic beaker) within the cistern below water level
to make it less susceptible to vibrations.

Hope this helps.

Cheers, Simon