Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Rotary Switch in CH and DHW position
Keep case switch depressed
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across terminal 4 (timer output) and terminal 1 (Blue wire marked N) on the main terminal block.
Note:
If timer not fitted then a link will be present between terminals 3 & 4.