Vaillant ecoTECH 637 - F.15 fault?

Boiler is 2 years old and has been working well. Heating comes on with timer and then goes off after about two hours. F.15 is on display - nothing about this in the manual. I have swapped a couple of radiators recently but system has got good pressure and there aren't any leaks. Cannot figure it out and though a plumber balanced and bled the system about 2 months ago no other works have been done with the heating - Any clues