That was interestng. I just had the 3-port valve replaced (not by BG - as they will not cover this on my contract due to sludge and have already said that the PF had broken it). The boiler stat has also been replaced (BG said there was nothing wrong with the stat).
The PF people have just called and tested the rad water again and told me (as they would no doubt) that it is clean.
I have cancelled my BG contract due to, 1) Forcing me to have a BG PF at a cost of £700, 2) Telling me that the PF had broken the valve, 3) Being lied to that a BG PF would include a new valve, 4) Not testing the water to see if sludge was an issue, 5) Too expensive. Basically indefensible and incompetent behaviour, and I will take my custom elsewhere.