UK Vented Heating Problem - F25 Error on Glow Worm Boiler
Looks like quite a few people still come across this question so I thought I would post what the problem was in the hope it may help others.

After trying everything it could possibly be, including replacing the thermistor in the boiler, in the end it was a good old British Gas engineer that fixed the issue with a couple of air release valves (automatic bleed valves) on the two bleed points in our airing cupboard. The air was getting trapped in and around the pump and valves no matter how much manual bleeding we did. 

The end result is a working central heating system, and the odd short sharp hiss coming from the cupboard once and a while.