Follow up - have rejected the cooker and Comet have refunded us the full purchase cost.

 Curiously, (and unsurprisingly given their previous appalling customer service and inability to retain the simplest information) a week after receiving the refund no-one has contacted us to arrange collection of the faulty cooker...

 I have asked them to collect it, but.....nothing!

 The refund letter was sent to an address I've never lived at! (although thankfully the actual money went into the right account). The original complaint has been escalated with Trading Standards, who have passed it on to their office local to Comet's headquarters in Hull.

 COMET - you truly are the most useless, incompetent waste of space I've ever had the misfortune to deal with. And I've had to deal with Southern Electric Phones, so that's really saying something. If you can't cover your own warranty obligations you should not be selling the items in question, or you should not claim to offer a warranty.