Chescold thermostat playing up on gas

Hi All

I've got a Chescold F400 3 way fridge.It works well and will freeze items overnight.
On 240V, it will keep a desired temperature, depending on where you set the thermostat. On gas, the position of the thermostat doesn't matter, as it just keeps chilling- hence it will freeze everything if left running on gas. Even when the thermostat is turned to the off position it keeps running!! Obviously with it like this I have to turn the gas off at night to prevent everything being frozen the next morning, but apart from that it works well.
Before i take it the get fixed, I was wondering if anybody knew what could be wrong. They look pretty simple, yet the parts are expensive, so would rather have a crack at it myself.

Cheers