OK,

I am wondering if my motorised valve might be the problem... please
see point 2 below.

Last w/e I went out and bought 2 litres of non acidic descaler, added
these to the system via the header tank (perhaps my system is not
sealed as per your definition Ed!) and flushed it through to make sure
that the descaler got into circulation. I also gave all the rads a
good bleed

The banging continued ( I am not sure how long it takes for the
descaler to work but presumably a while as it will only work on
contact = thick layers of deposition will take a while to go).

Couple of days ago I bit the bullet and fitted a replacement pump -
got a Grundfos Alpha+ 15-50 as every one seems to think they are good
here. The noise has definitely reduced. Now there is perhaps one
bang (was 3/4) and no or little pipe groaning building to the bang.

However there is still a bang. Diligently I sat by the boiler at 6.30
this morning waiting for it to come on and notice two things

1) When it did come on there was one medium bang and then it sounded
like the water flow through the system really took off (lots of
whooshing)

2) Before the heating came on the HW circuit was energised but not
running yet (HW tank stat was not calling for heat) All the pipes
were cold (ie bolier had not run) BUT the valve cover and body was
really hot - say 45C or so yet the water temp in the pipes was
ambient. Would you expect the valve to run hot like this? can the
valves fail in this way and...could the valve also be responsible for
making the flow sluggish on system start up hence the banging???

Thanks again for all your response - very much appreciated

Tim