glow worm 30ci fault f5? anyone?

i have a glow worm 30ci combi boiler which is flashing a red light and reading f5 on the indicator does anyone know why? i have read the manual from cover to cover and only tells of f9 f1 and f2 according to manual f5 doesn't exist, glow worm have been really helpful (not) by telling me for £210.00 they can send an engineer who will be able to tell me, can anyone tell me without ripping me off in the Christmas season?