Yep! "Reject" is what I intend to do tomorrow. We will demand a full refund as we're worried that if they simply replace the cooker with the same model, there's no guarantee that it won't have the same problem.

 Then I intend to go to a specialist Calor centre and buy a cooker which is already set up for LPG. I will however follow this up with Trading Standards (already lodged my initial complaint) as if they sell LPG cookers and don't have any LPG engineers, they cannot possibly perform their contractual obligations under the warranty. 

Cheers.