What have I done wrong
 

I have just put a new clutch and exchange gearbox in, got it all back together. All good so far took it of the stands still good, went to push it and its in gear no problem jump in car and take it out of gear, BIG PROBLEM the gear lever won't go forward or back. What have done/ not done?

Check your linkage at the transmission for binding, disconnecting, or being upside-down.