The gun shop (Dolphin Discount Stoke on Trent) have been great, appologetic and asked me to bring it and so they could return it to AA.
AA also contacted me very quickly, they say that the only time they have seen this problem is when the cylinder pressure drops below 100bar. As mine had only shot a handful of pellets from a full charge it wasn't likely to be the cause. However we agreed amicably that I will continue to use the gun as it seems to be working ok again at the moment, if the problem does happen again I will send it back whilst faulty.
I still think it has the potential to be the best hft gun out there as supplied from the manufacturer, meaning you don't need to spend hundreds of pounds extra to get the gun to fit the individual. Of course the fit means little if the gun is likely to fail but I am prepared to give it another go, for now!