If you must spend it right away and the tank in your sig is your only tank, which is pretty near maxed at the moment I'd go for Tabbiespice's suggestion, a betta and tank, otherwise keep the money to put towards replacement fish later or new equipment for another tank if your going to go down the MTS road
My lfs sells 20 us gal/15imp gal tanks for £20 (24"x15"x12"). A tenner for a heater and a DIY filter till you can buy one down the line. sold.
Personally Im awaiting an extension being built to splash £50 on a 48"x18"x15". think i may drool a little when i get it. Though It'll probably be about £150 by the time i add my lighting system etc
What size should we look for for a betta tank? about 5 US gal?
I've got a local shop selling 12"x8"x8" for a tenner inc hood. but for £1 more I can get a 16"x8"x8"... though it obviously requires more room... but for a quid, i think its work it. would a simple air activated sponge filter be adaquate for one?