Glowworm Flexicom F1 F4 faults?

Having a problem with boiler cutting out. NOT a condensate problem. Key bit of information is that it will run in central heating but after a hot water tap has been used it won't continue with central heating, get F1. Also, if when running in central heating and boiler water at say 80deg the temperature is reduced to 75deg ( so that boiler cuts out) then increased back to 80deg ( so that boiler tries to come back in) get failure F4. What could cause this?