How can I fix my Volkera Mynute 28se boiler?

The timer on the wall will light up in Auto but the boiler will not ignite. I have tried switching off the system, pressing the reset button, then switching it on again. I have also tried switching over to "All day mode" and sometimes that works. However yesterday, the boiler eventually ignited in "All Day Mode" and then after approx 30 mins, the system switched off. (The timer was still showing as on at the wall though). The pressure when the system has ignited is good, well over 1 on the gauge.