Top of the mornin' to you Miaow.
1) Shop your local bass stores, then before you buy go here:
http://www.thomann.de/gb/index.html
This of course is online, so you can't try the bass... but you will know what to pay for one and get a great overview of what's out there.
2) Fender's second brand is Squier and they make solid instruments. Thomann lists a Squier jazz bass for £245 which is very reasonable.
3) Considered used as well. My first bass was a Fender Jazz in very good shape for a reasonable price from an online auction.
4) Heads up: a basic decision to be made will be choosing between a P-style and J-style (jazz) bass. P-style will be a little more low end with a fatter neck, J-style will give you more mid-range tones (putting you further in front in the mix if you play with a band) and a thinner, faster neck. Each style has different pickups.
5) Here is a good introductory article on Wikipedia as well:
http://en.wikipedia.org/wiki/Bass_guitar
Finally, to echo a point made above, don't go too cheap, but instead look for something that feels, looks and sounds good to you... you will enjoy it every time you play.
Enjoy, let us know what you decide!