Don't count on quick raises from a very low salary unless you've explicitly discussed that with your boss or at your interview. In most places I've seen, they're happy to calculate raises using %, which means you get a whopping 20% raise on a £20k salary - which still amounts to peanuts (£24k is not much different from £20k).

Negotiate your starting salary as high as you can, and don't count on quick raises.

Here's a useful article on the topic from ever-helpful Rands:

http://www.randsinrepose.com/archives/2008/04/11/the_busines...

Daniel