See this page that lists Dell deals
http://www.dmxdimension.com/dell-uk/inspiron-660-deals/
Select the Insprion 660 desktop (note the 660s is the slimline model).
Scroll down to the £349 model that has an i3 2120 CPU and 4Gb and 500Gb hard drive (or the £359 model that has a 1Tb drive)
Then notice near the top the codes you enter to get 12% off AND free delivery. This brings the price down to £317.
They will send you a mouse and keyboard but for some reason most companies do that.
Pretty good deal for a desktop. Note the 12% offer may go soon so dont hang about.