Potterton Netaheat 10/16 Mk1 main burner cycling on start-up?

Can anyone help with this problem?

When the boiler starts, the fan starts (normal), then after a short delay (normal) the pilot light activates (normal) and then the main burner ignites (normal). However, the main burner only comes on for about 3 seconds and then turns off and then comes back on again. The main burner igniting on and off is a continuously repeating pattern. The voltage reading across T1 and T3 on the control box (what supplies the solenoid valve) also cycles between 0V and 240V in tandem with the main burner cycling. The control box is new.

Any ideas?