Been riding round on me bike and back cog has come loose could anyone tell me how to tighten it back up.

Thanks

There are a variety of special tools used to tighten the cogs into place. Which one you need depends on what you have -- i.e.,, Shimano cassette, Campy cassette, freewheel, etc. Easiest thing to do is to go to your LBS and ask them to tighten it for you, or at least tell you which tool you need.

cheers kevin