if it's in the 3 year warranty it's free to send it to microsoft, otherwise it starts from £30 and shoots up towards £100 for repairs by Microsoft. there a guy in wallasey that can repair them cheap if its out of warranty. you can see his details and such over at http://www.allconsolerepairs.co.uk/xbox+360+repair+service.htm
one of my associates had his xbox 360 repaired here ages ago and it hasn't had a problem since.
personally i've given up on the 360; too many hardware problems (i've been through 4 one of each chipset) and the software variety is small.