Thanks. I have never spotted one of those and suspect I would have noticed it. If they block after 2 years then I doubt I have one as I have run this system since I bought the house in 2006 and its had a new pump and a new diverter valve motor in that time (plus services).
Well, what a saga.... So... British Gas were supposed to come yesterday. They rang at 7pm the evening before and said 'we are ringing to re-schedule your appointment'. I was firstly confused and then angry - it had been booked for four weeks as the earliest appointment I could get. Further to this, I am away for another month from tomorrow and their next appointment is next week... Not useful. The boiler hasnt been used since the start of April and my lodger will get frustrated if it isnt working again soon.
After some arguing, they said they would pay if I got another Gas Safe Engineer to come round.
My neighbours know a nice chap so I text him on Tuesday evening and he was round after lunch on Weds. After a good look at everything and a few checks, he concluded it was imbalance around the air valve (hope that makes sense?) and after blowing through the lines and restrictor, checking the flue, renewing the heat transfer paste on the boiler stat sensor, the boiler is now behaving. Someone had also managed to break the thermostat since I originally posted (the knob wasnt turning the spindle at all - it wasnt like this when I last posted) so he replaced that too. He finished up with a proper service and the landlords checks.
I think I might just pay him direct and cancel the BG Homecare. He knows the system now and got to me a lot quicker than BG have in the last few years. So I can get him to do the landlord checks because I have a lodger etc and future services.