ShortScale.org

Apart from ebay and gumtree what other options are there? Im thinking about a Les Paul in a few months and have decided to go used.

i'd recommend sound on sound, i've bought stuff from there before. it's kind of geared towards recording and programming hardware but there's always tons of guitar stuff for sale too. http://www.soundonsound.com/readersads

a quick google search throws up these:

http://www.loot.com/

http://www.adtrader.co.uk/postcode_splash.php

http://uk.freeads.net/

there are also plenty of dealer websites with used gear, but you're generally looking at a 100% markup compared to street prices.

http://www.preloved.co.uk has turned up a few goodies too