Faulty Leisure Mistral EL?

This is a manual light Gas Cooker, the hob is fine but the oven section has just stopped heating up. The gas comes on and will light but it does not turn to full flame after a few seconds like normal. The 'holes' where the gas comes out of the burner at the botton are clear and I have scraped some burnt 'stuff' off the cross piece that looks like a mechanical sensor (there is no power supply to this cooker) there is also another mechanical sensor at the top above a metal plate. Any help would be appreciated.

Thanks