Just had the Aldi Christmas book and their magnum of prosecco is £12.99 in store 13/11 order online now and free delivery on it too.
I love magnums as you don't feel as guilty about technically having 2 bottles as it's only one actual bottle and they look awesome when you give them as a gift too.
Asda fillipo sansovino prosecco magnum - £18
Morrisons Menestrello prosecco magnum - £14 (until 27/11 then £19)
Tesco plaza centro prosecco magnum - £14
Sainsburys coneglaino prosecco magnum - £19