The original is (apparently) two parts orange juice to one part Champagne.
The Chatham version is half a glass of Lucozade on top of which is poured half a glass of Cava. I gather it's important that it is done that way round. The result is presumably 'stirred, not shaken'.
Hmm... I don't think I'll bother with it myself – actually I don't really fancy the original either – but offer it as a way for even the chavvy folk of my home area to participate in the idea at a more readily affordable cost.