Thetford hob mystery fault.

Here is a three pipe problem for Sherlock out there to resolve. Last year we toured Europe for 5 weeks with our 18 month old van. The Thetford gas hob rings would light fine but if we released the knob the flame would go out so we had to hold in the control knobs for about a minute before the gas ring would stay alight. Once lit they were fine and all other gas appliances worked as normal although when we got back to UK I noticed a smell of gas in the front locker (faulty cylinder valve or connector?).  Somebody on this forum suggested that this was a Thetfod design problem and that shorter control knob shanks were needed.  I pressurised my dealer and am due to get this checked under warranty at a service next week.

Now here is the mystery.  Collected the van yesterday from storage, connected a (different) gas cylinder, fired up the Alde, checked the hob and, yes, you  guessed, absolutely no problem!  After two seconds the gas knobs could be released and the rings stay alight.  I am an engineer but no way can I explain that.  The only difference is that it is freezing outside (was hot in Europe) and no gas smell in the front locker.