Problem with Vaillant TurboMax VUW 242E, fan/boiler not running?

My boiler had divertor valve problem in past, plumber replaced it for a reconditioned one-it lasted about 2 months. The hot water was ok when heating was on. I decided to live with this.
Several months on the boiler has stopped working, the red light at the top is on and the yellow CH demand is glowing. On the main control board there is only 23Vac instead of 240Vac according to the service manual, and it suggests the main circuit board is to be replaced. Disconecting the NTC temp sensor and restarting the boiler made no difference.
All fuses are ok. I am an electrician so I will do it myself, before I replace it is there anything else I should check.
If it is the circuit board is there a known component that can be changed/tried. The fan appears to be powered by a relay whose coil resistance seems ok. No burnt components visible.