Personally, I'd keep the 10-20 (had one on a 40D for a couple of years and loved it) then maybe get a Tamron 28-75 f2.8 (used) for a walkabout (one for sale here on TP for £230 which is an ok price) and then treat myself to a Canon 200f2.8L II which you can get for about £400ish, again used.
Just about on budget and you will have three cracking lenses.
Just my 2p worth.
Gary
Thinking of canon17-55- good idea? Thanks for all suggestions