Look for socket 1155 motherboard and CPU combinations. That's an Intel specification, rather than AMD.
Most up-to-date motherboards require DDR3 RAM. Most likely, you currently have DDR2, which could not be used.
I've seen socket 1155 motherboard/CPU combinations for as low as $180. Add maybe $25 for 4 GB of DDR3 RAM.
A mid-range socket 1155 motherboard/CPU combination might be $300, plus RAM. At $400, you would be getting a very powerful PC.
The choice depends mostly on what you intend to do with it---game, write a novel, make an H-bomb, etc.
I realize you are in UK, but here are some US sources to give you an idea. You can shop for the same stuff at Amazon UK or whatever supplier you have in mind.
Here are the socket 1155 CPUs at Newegg. A midlevel one would be $100 to $200. Newegg.com - Computer Hardware, CPUs / Processors, Processors - Desktops, LGA 1155
Here are some motherboards for socket 1155. I'd get Asus, Gigabyte, or Intel. A price around 125 to 150 would give you most of the worthwhile features. Newegg.com - Computer Hardware, Motherboards, Intel Motherboards, LGA 1155
Add $25 for 4 GB of DDR3 240-pin RAM.
I'm assuming your case, power supply, graphics card, hard drives, and operating system don't need to be replaced.