How can i fix my combi boiler [SIME, FRIENDLY FORMAT 80 E]?

When i put on the central heating, it works perfectly for approximately the frst half hour (i.e the radiators heat up as normal). After approx half an hour, the radiators start to cool down. When this happens i have noticed that the boiler is still on (i.e. the fan & pump are working) but the burner goes off. The burner does not come back on by itself. To get the radiators working again i have to switch the boiler to 'off' position first, then on the 'on' position.