Ok, so today we are feeling a little silly!
Rather than following our hire companies instructions to return the vehicle to Banbury for an inspection RE: gas problem, we decided to call in to a local caravan shop as it was on route. A couple of very helpful gentlemen came out to assist and located the problem instantly. It turns out that there is a saftey cut off switch on the hob lid which kicks in if the lid is not 100% vertical - DOH.
Despite looking a little silly, we are pleased to have resolved the issue without disrupting the run with a trip across country!