The pilot light is on but the burner won't lilght

When the oven lighting mechanism lights the gas the flames heat a temperature sensor that, once hot, increases the pressure in the sensor's copper pipe and opens a safety valve to allow gas to continue to flow (It's made like this so, if your flame gets blown out, the sensor cools and cuts off the gas supply, making it safe from gas leaks). These sensors occasionally fail, and very sensibly "Fail safe" - No pressure in the sensor pipe, the valve won't open and there is no gas to the burner.

If you take a look inside the oven at the back where the burner is, you will see the sensor, usually the bit that is in the flame is cylindrical, about an inch long made of metal about 3 or 4 mm thick, tapering at the end. It will be held in place by a nut that fits on a thread at the base of the sensor. Running from the sensor is a copper pipe that runs to the cutoff valve. Unbolt the sensor and withdraw it from the hole it is mounted in, trace the copper pipe back to the valve (where it is usually screwed into the valve with a 10mm connector that looks like a car brake pipe connector. Unbolt from the valve. These sensors are widely used on most gas appliances (Boilers, gas fires, cooker hob burners and patio heaters all use the same kind of sensor). You should be able to get a replacement from any gas appliance service agent. If you go into a shop, take it with you. The only thing you need to do is make sure the replacement you get is long enough. If you are offered one that is slightly longer, that