Ginger ale and lime cordial, topped up with soda water.

I'm drinking a lot of bitter lemon at the moment, but I'm mixing it with whatever fizzy wine I pick up in the supermarket, but you could try something suitable as a blend?

Also, I have heard a friend speak quite well of some non-alcoholic gins, if you wanted something more down the juniper and botanicals line - which makes sense, really, as gin is just flavoured spirit. So have the flavour and not the spirit!