I have an outside tap connected to an external blue pipe. The connection is a compression type (I think) I have unscrewed this and pulled it down the pipe a bit and now I can't push it back up the pipe to screw it back on. Help

If the compression nut slip down the pipe it should slip back up the pipe unless you have done something to the pipe. Try working it back up and if that down work take a pair of pliers and put them loosely around the pipe at the bottom of the fitting and see if you can tap the pliers and the nut back up. Let me know if this works for you. Good luck, Tom