Go to Micromania on Mikarious Roundabout, you can get a brand new full suspention bike for £100 called Bullet. Will save you hassle in the long run. If you buy a cheap 2nd hand one it most likely will cause you problems if it's not been looked after.
I bought a GT Avalanche 1.0 (Disc) from there about 2 months ago for £400 i use to ride to work from Akrotiri to behind the zoo on the strip, if you doing that kind of distance i would recommend something more pricey. But i also do off road and downhill XTC so upgrading the bike is essential as basic parts wont last. I also recommend getting some hope hydraulic disc brakes too (around £200 each)
Here is the website:
http://www.micromania.com.cy/biker/