To BG MIKE or anyone !! (re. [2] burner and pilot light)?

From cold this morning, the boiler ran for a good half an hour, BUT with the thermostat on the very lowest setting. Once I turned the thermostat up (ever so slightly) the problem reoccured.

I tried it on MIN again, but as the boiler was already hot, it only ran for about two minutes, and then the burner and pilot went out.