Got a gas hob in a caravan which is fired by pressing down the knob, lighting it and then keeping the knob pressed down for approx 5 seconds. When you let it go it keeps going.

Only one ring is now working, the rest will burn but cut out when you let go of the knob.
Is it likely to be the gas tap part, thermocouple, the burner?

Any ideas?

It's this one knackered cooker

I have that on one of my burners in the camper van. I found just leaning on it for longer made it stay on.

Ive no idea how they work though – voodoo?

Its like voodoo, only darker

Thermocouple heats up in the gas flame and creates a current that holds out a magnetic valve in the gas tap…so if thermocouple is stuffed, as soon as you let go of the knob, the gas is cut off. This is a safety feature so that van is not filled with gas if the flame goes out. Thermocouples do not last forever, so that would be my first guess – maybe you are not letting it heat up enough …hold knob in for longer.
Failing all of that, you can replace by taking out old one and toddling along to you nearest decent plumbing and gasfitting supplies shop. You may have to buy a Universal thermocouple Kit. BUT check if the insurance on the van says anything about non licensed people working on Gas parts – may be an issue!

Cheers tegski

Hit the nail on the head. Swapped thermocouples between working ring and non working and it works.

Excellent – last job and five years in the gas components industry was useful then! Happy cooking!