Hi, at the moment I am using a sigma 18-250mm 1:3.5-6.3 and a tokina 12-24 f4 dx.
I'm going to be honest, and dont know a massive amount about glass, so any advice would be appreciated
First bit of advice, get rid of the 18-250! As a general rule, you always have to compromise between flexibility, and performance. Primes are least flexible, but can give stunning sharpness and speed, superzooms like yours are very useful in some situations, but trade off optical performance as a result.
What I'd do is get a Tamron 17-50 f/2.8, and a Sigma 70-200 f/2.8, and perhaps a x1.4 teleconverter if you're missing the extra length. That'll replace the focal length with much faster, sharper lenses, and that really will make a difference (used correctly of course).
Second hand that'll set you back about £600, or £700 with the teleconverter. Always leave a bit of your budget left over for those extras you haven't thought of yet! Or if not, then you could probably find a Canon 70-200 f/2.8 (non-IS) which would take your right up to your limit.
If you tell us exactly what you shoot though, that could have a big effect on recommendations, this is a sort of general-purpose suggestion, if you're really into one particular type then it could be very different!
Chris