It is for a Mac running OS X Snow Leopard.
I want to able to run time Machine!
What is regarded as the best value £ for size?
My Desktop Mac has a 250 gb so need one that size
nah, you need one at least twice that size if you want to store any history in your time machine backup.
Tesco usually have some fair deals. Yes, you could go online and save a few pounds. But... I bought a bunch of Hitachi 1TB drives when they were on 'offer' at Tesco last year - about £54 each. Something like that would do you fine. Get 2. use one connected all the time at home and locked with a kensington lock to something secure for TIme Machine. Use the second one for SuperDuper or CarbonCopy or Backup to store a scheduled incremental backup of your users' folders. Take it into work/to a mate's house after you've done the back up. This way, you can roll back time any time with Time Machine. But if your machine is stolen or your house burns down you have some security in having an off site backup. There is a chance that your Kensington-locked TIme Machine backup might not be stolen if your desktop machine is.
You could go further than this if you're more paranoid about data loss.
Reformat any drive you buy to HFS+ using Disk Utility. And give them distinctive icons so you can identify them easily in Finder.