I'd be tempted to say
5750 512MB @ £85. For 1366 x 768 it'll be slight overkill even with everything turned up (except perhaps in Crysis

) but it prepares you for DX11 down the line, gives you that much better power profile than 4-series cards and is cheaper too. Oh, and should you fancy it later (and you don't mind shelling out for a DisplayPort equipped monitor) it'll cover you for Eyefinity
On the NVidia side the only "new" GPU in the same band is the GT240 - make sure you go for a DDR5 one
like this XFX (which incidentally seems to have a rather short PCB!). A GTS250 is basically a rebadged 9800GTX+ which traces its lineage directly to the venerable 8800GT, but is no performance slouch. Ebuyer do have
a 192-shader GTX260 available which also wouldn't be a bad option @ £107.
Personally, I'd go for the 5750.