We finally have an oven now. We ended up cancelling our order with Comet because they screwed up the original installation. When we went into the shop on monday to complain, they were unable to help. They couldn't bring the installation any sooner than friday. When I asked why the original installers were unable to determine the source of the leak, I was repeatedly told that 'You have to be careful with gas'. That would have been an ideal response if I'd asked why they couldn't just ignore the leak, but since I had asked why the engineers seemed incapable of doing their job properly, it really didn't answer my question.
We went next door to Currys and placed an order with them instead. They agreed to match Comets price and could deliver and install on thursday.
So this morning Comet came to collect their cooker, and Currys came to deliver and install theirs. The Currys engineer still detected a small leak, but instead of buggering off (like the Comet engineer did), he actually did a few tests to locate the leak. He checked the boiler and the gas pipe in the front room and they were ok. The 'leak' turned out to be the capped pipe in the kitchen - the man from Transco hadn't sealed the cap properly.
With the source of the leak located and fixed, the oven was installed. After demonstrating the controls, the engineer left.
The service I received from Currys and Comet couldn't have been more different. Comet did a half-arsed job and were incapable of rectifying it in a prompt manner. Currys went out of their way to make sure the problems were sorted. Currys might be slightly more expensive, but their customer service seems to be much better.