There is one you can get in some supermarkets and from health food shops. It's not diet but it is cloudy and still (ie not carbonated), I am trying to remember the name of it! It begins with a P (Pottingers? something like that) and comes in funky, small, glass bottles, it is quite dear about 2 quid for a bottle.
Also, Rocks Organic Lemon Squash is very good and will probably deal with your craving, although, again there is no diet version.