How do I get my heating to stay on until i manually switch it off?

I have a Worcester-Bosch Greenstar 28i junior condesation boiler, which has only been fitted for a year. However a few months ago, I have noticed that when I turn on the heating, while it heats up for approx. half an hour, it then automatically switches off and the ignition light goes off, even though the power supply remains on.

Eon had a promotion on where they insulated and fitted out 100 homes called Challenge 100. I did not at that point ask how long - or even if there is - the warranty will last for the boiler, or whether I will now be out of pocket. Also was not provided with a manual or guide, to see if perhaps I may find an ignition button or whether it is on the boiler, but I cannot see it! Will be grateful for any replies!!