ideal indipendant c24 boiler keeps flashing F1?

Our ideal c24 boiler keeps flashing F1 every time the heating isn't used for a while. E.g every time the house reaches the right temperature and the boiler clicks off for a while the F1 light flashe up on the display!? Help. The only way we have got it to work is by turning the boiler off at the plug, opening the valve by the pressure gauge and turning the boiler on... After a minute if we close the valve by the pressure Gauge the heating starts to work again... However this only works some times and takes us about 6 attempts for it to stay on as usually as soon as we close the valve it flashes f1 again!!! Please help!! Thanks in advance!