An open circuit 47 ohm resistor between the live mains and the bridge rectifier meant no power getting on to the board. Replaced and now sorted. Thanks for your help.
Power measures fine on the back panel of the control. Boiler switches on for hot water. Is there a separate fuse for CH in the boiler (Worcester Bosch Greenstar 28i Junior)?
Initially the display disappeared so I cured it with a replacement back up battery. Now neither the timer nor the ADV or EXT controls will switch the heating on. The control beeps and the little LCD indicators come on but the red 'ON' light does not illuminate. Any help greatly appreciated.