Thankyou for your helpful responses. Please forgive my followup
questions - they will probably sound really stupid to those who have a
plumbing background - however I am learning by doing, and learning
fast!!! So here goes....

1) When we flushed through the system (weve done this about 4 or 5
times now) the water now runs clear. Where would the blockage be
occuring if it appears that water is running through the system?

2) Why is it that a blockage causes the other symptoms of banging and
crashing in the system? Im afraid that something has gone wrong inside
the hot water cylinder or between the boiler and the cylinder. The
cylinder also seems to get very hot - probably overheating.

3) Do you have any instructions as to how to 'back flush' the pipes.
Would I have to connect a hose pipe and flush through mains water?

4) How would I collect the water with inhibitor in it?

5) Would there be any point in changing the pump even though it sounds
like its working?

6) Should we just bow our heads to the inevitable and pay out for the
powerflush as first advised?

Again, all advice very gratefully received.

Coldtoes.