Your boiler's controller has lost track of its timing schedule, which can stop heating and hot water from switching on and off at the right times, but this is often fixed with a simple reset.

DIY fix possible

Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 2 minutes.Switch the power back on and check the boiler's date and time on the controller display, then reset them if they are wrong.Check that your heating and hot water timer programmes are set correctly, then see whether the boiler starts working normally and whether F.450 returns.