Ok Jon,
On the basis that you have had to ask this question, I have to assume you are not a qualified gas fitter. For the safety of you and your family, refer all maintenance, servicing and repairs to a qualified gas fitter.
Also having asked the question I assume you don't have the handbook for the appliance which leads me to suspect the caravan is not new and probably second hand. This begged the question of when it was last serviced...
As other appliances are apparently working ok, the problem with the grill is unlikely to be the gas supply. So it suggests there is a problem on the appliance. I am not familiar with detail of Spinflow cookers, but most modern grills have a flame failure device FFD which is designed to cut off the gas supply if the flame goes out.
The lighting instructions will almost certainly tell you to turn the valve control to the ignition position. push the valve in and hold it whilst you operate the ignitor. Once the flame has established keep holding the valve in for between 3 to 10 seconds to allow the FFD to warm up. Then release the valve. The flame should remain alight, and you can then turn the valve to regulate the size of flame.
If the flame goes out, then it could be due to a number of things such as FFD failed (to safe condition), incorrect flame pattern or size. Apart from brushing the burner holes with an old tooth brush to remove any debris, anything more really needs a competent fitter to investigate.
Even though a gas appliance may not actually be used for a length of time, they can still deteriorate, Insects quite like the small crevasses inside gas appliances and a spiders web is quite capable disrupting the gas air mixing inside the burner, leading poor combustion often typified by candle yellow flames.
Incidentally, when you say "last year so they said" If that referred to a general caravan service, then be warned, that does not usually involve a full service of the gas appliances, Normally they would check the gas tightness, the visual condition of the appliance an that it appears to work. You normally have to request a service on any appliances in the caravan.