Ok, I'd say you want to try to match your front three, as they're responsible for 85% of a 5.1 soundstage on average. You haven't said whether you've got a br or bx centre. Assuming the latter, a pair of bx5s would be the natural match for your centre and should only set you back £450 or so. An alternative would be a pair of kef floorstanders, probably the excellent IQ5SEs would be the best option, again these should cost around £400. I'd recommend trying to get a demo of both. One word of warning. If you go down the bx5 route, you may find that you lose some cohesion on steering the effects around the soundstage. You might want to consider changing your rear eggs for a pair of bxfx in this case - which will still be within you budget. If you go down the IQ5SE route, you might want to consider getting a new kef centre - not am egg, but one of the chunkier ones...
EDIT ok, I know the iq5se is a discontinued speaker, but I feel they are better than the iq50 and you can still get them at the price I quoted from Richer Sounds (say)