Dimension 2400 for £155.16 including VAT and delivery. Dell is offering free delivery until the 15th of December and there is a 5% off code for the Small Business section (anyone can order there).

Specs:
Dell Dimension 2400
2.6GHz Celeron Processor
256mb DDR RAM
80GB hard drive
48x CD ROM
Windows XP
1 Year Warranty.

Follow the above link to Dell Small and Medium Business. Scroll down and search by e-Value code 305-1221h. Change support to 1 YR .Enter the discount code ?7NWS3?Q45F?M0 at checkout.