Phil L wrote:

Thanks for all replies, and this is my plan of attack, prior to hanging new
boiler:

1) take off all rads and blast them through with hosepipe, this will take a
few hours but I'd rather know for certiain that they aren't full of
'treacle', like they were last time.

2) hang all rads and refil the system adding a flushing additive, run
through on a low heat for 30 minutes, or according to manufactuers
instructions.

3) Drain system via bleed valve near front door, and rinse thoroughly.

4) Refil system, with added 'fernox' or similar inhibitor.

5) Hang new boiler.

I do realise that plumbers now offer a powerflush, but last time I looked it
was £200, and even then it was only as part of a major service or fitting of
a new boiler.....I reckon the flushing crap will set me back about £20 and
the same again for the inhibitor, so I'll save at least £150, and as an
added bonus, I get to play in dirty water for a day.