Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Central heating temperature control set to MAX
Mode selector switch to CH position
Insert a link wire between terminals = (1) & TA (3)
(as indicated by the red arrows)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the burner ignite ?
Note:
If no external controls are fitted ensure that a link is permanently fitted between terminals = (1) and TA (2)