As has been said, if you are brewing using sugar (table or brewing) try switching to Dry Malt Extract (aka spraymalt) and reducing the brew length from 23L to 20. This will make a noticeable difference in flavour.
You could also try dry-hopping - adding 30g or so of some appropriate hops (East Kent Goldings always work well) to the fermenter after the 4 days or so, and leaving them in there until you keg/bottle it at 14 days. This will give it a fresh hoppy nose and flavour.
I would avoid doing two bitter kits together though - it's likely to turn out too bitter as the kits were designed to be mixed with either sugar or malt, and already have enough bittering for the full brew.
