Tesco have responded to the recent supermarket price deals on champagne and are selling the following at half price until 28th November (all non-vintage):
Taittinger Brut Reserve - down to £16.99
Heidsieck Monopole Blue Top - down to £13.99
Lanson Black Label - down to £14.49
Mumm Cordon Rouge - down to £14.99
They have others at half price too, but they are the co-operatives and BOB ones (Nicolas Feuillate etc.) which are fourth or fifth pressings and are poor quality compared to the above (and are usually at so-called "half price" at Tesco anyway)
I saw the deal in a small local town centre Tesco and snapped up a load, then went to a big out-of-town Tesco and although the shelf edge showed full prices, when I scanned the bottles they came up as half price. The shelf edge prices in the little store correctly showed half price, and said the deal ends on 28th Nov., so be quick as they will fly at this price.
Just check the price at the till if the shelf doesn't show the deal.