First GTX 580 I've seen for <£400, and it's an Overclocked one!
This weekend Overclockers UK are posting up some deals at http://forums.overclockers.co.uk/forumdisplay.php?f=115.
Having a look on other e-tailers websites I can't find a GTX 580 for under £400, and the nearest ones I can find (but over the magical £400) are own-brand ones @ stock speeds, whereas this one is a MSI card and is overclocked!
Remember that forum members get free shipping too :)
Prices go back up at Monday Midday.