How can I fix my Baxi Bermuda 57/3M back boiler? ?

My Baxi Bermuda 57/3M back boiler doesn't heat the radiators. The boiler flame only lights up for approx 1 minute then cuts out for 15+ minutes before relighting. the pump continues to pump the lukewarm water round the rads.The pump is controlled by a room thermostat. The orange boiler control is set to max with no effect. it can be induced to ignite by turning the orange knob to min then back to max. But this still only lasts a minute. The pilot works fine & remains alight at all times.