Asda Lanson Black Label Champagne, half price. £14.47, down from £29-30. Instore only (as far as I can tell), may be online later.
Believe it was the top rated champagne by Which magazine in a recent taste test.
There was an advert in one of the daily newspapers today (The Sun), I believe. One of a series of good booze deals at Asda currently, including 1L bottles of spirits and various beers offers.