Hop along to the Dell outlet, they have some good deals on the dimension range, e.g.:
Dimension 3000, MT, Pentium 4 2.80GHz, 533fsb
English - Win XP SP2 Home
System Price £324.79
Dell recommends Microsoft® Windows® XP Professional
Operating System: English - Win XP SP2 Home (Backup Media)
English - Win XP SP2 Home
Memory: 1024MB (2x512) Dual Channel DDR333
Floppy Drive: No Floppy Drive
Hard Disk Drive: 80GB IDE Hard Drive (7200rpm)
Modem: V.92 Data Fax Modem UK
Software: Microsoft® Works® 7.0
English - Adobe Reader 6.0
BT, AOL, Tiscali ISP Combination
MISC: Dell Standard PS2 Keyboard
Dell 2 Button PS/2 Scroll Mouse
DMX Dell Media Experience 2.
16 X DVD +/- RW
That's £324 ex VAT and delivery. Basically, anything on there that's under £330 will fall inside your budget.
This looks like what I,m looking for.