Well, I went and had a rant at Comet today. At first the store manager couldn't be bothered to come out to me, and I had to see the Customer Service assistant.
For some reason, he was unapologetic - I didn't deserve an apology because of my "attitude". Of course, that attitude was nothing to do with them and their incompetence - I should just have come in and been nice. So I had him send for the manager, who then discovered that he's actually already had my email to his Head Office to deal with, and was a lot more accommodating.
So the deal - dictated by me - was the internet price including the 5% discount, all the free bells and whistles from before, and a date and time to suit me.
They agreed, though they say that they cannot guarantee the delivery time - their afternoon slot goes from 2pm - so I have had them mark up the docket as "not home before 4pm". If it actually arrives - Result!
If not, back to the drawing board.