Right I have a fox dhx 5.0 air shock, I have read the manual but it does not say what the 2 valves are for there is one on the end of the piggy back and one near the rebound dial. Can anyone explain what they are both for, I am 13 stone and not sure what PSI I should be going for in both. Cheers

The one next to the rebound is for the main chamber – use this to set your sag level. The one on the piggyback is effectively the compression. Take note of min and max pressures – 125 – 200psi. The blue dial around that sets the bottom out resistance.
There is a vid half way down this page that may help too. It is specific to the RM Slayer but the info about the shock may help.
http://www.pinkbike.com/news/Rocky-mountain-slayer-sxc-90-preview-2008.html