Pleeeease don't forget to let us know what happens. You could become a forum hero if you play this right
Leave it for as long as possible (several days) before releasing the pipe. There is usually sufficient seepage round the pipe to allow the pressure to drop.
Answers to some technical queries at http://coxengineering.sharepoint.com
Thanks everyone for all the answers as pvb said it wasn't as exciting or hard as it could have been. By working the diverter valve backwards and forwards a few times, it released any pressure in the outlet. It turned out to be a blockage in the stop cock after the ball valve, so with a length of 1.5 in tube to act as a coffer dam I could open the valve and poke the blockage out from the inside. Problem solved!!