Cannon Chichester - takes ages for full flame to appear?

Similar question to several others - but in my case the (main oven) flame DOES eventually work, but only after it's been been on the "pilot light" for around after half an hour, and then only when it's been turned off then on again. You can tell it's going to work when you turn it off, because the flame gets momentarily "high" just before being extinguished, if that's a clue as to what could be wrong...

Thanks in advance...