Thank you @Exedon and @oilhead. You were right about the photocell. Fitted a new one and boiler's working fine again (touchwood). Really appreciate you taking the time to help me out.
External Grant Vortex Pro boiler works when first switched on. The radiators get hot and all seems fine. It's only used for central heating, not hot water. The boiler switches off after about 30 minutes. After a few minutes it restarts but the problem is the Riello RDB1 Vortex 15-26 burner does...