Leisure 75 Gas Cooker (with Eyeline Grill), Oven not heating up?

This is the very basic Leisure 75 gas cooker with manual gas lighting. When the oven is lit it usually starts very low (as the S setting on the otherwise numeric dial) and then after a minute starts to burn at a higher rate until the correct temperature is reached. My oven starts at the low rate but doesn't go to the high burn even after being on for a considerable time.