Put the gin in a jug with the ginger cordial and the lemon juice. Chill, covered, until needed (for up to 48 hours). To serve, pour about 3cm of the mixture into champagne flutes (or 2cm into coupes) then top with chilled champagne.
Garnish with lemon peel and serve.
This has a fiery kick of ginger so, if you’re not keen on that flavour, substitute with a good quality fruit cordial of your choice.