The boiler is actually staying on for a while now so I think the fernox f3 has at least cleared some sludge out of the boiler in the past 3 days. I am still having banging and gurgling in the pipes. I plan to leave it in for a week, then drain, mains flush with a hose and refill, then repeat with another bottle of f3.
Does that seem a good idea?
With regard to mains flushing, I was thinking of disconnecting the return valve on hallway radiator and connecting hose there to tap, while draining from drain point in airing cupboard via another hose.