Thanks for the tips guys. Tried a few things this morning and nothing helped so I got a mate to check it out before we started to run in the new motor.
Well it seems as though it was a combination of a few different things. Firstly, we had to put little rubber spacers between the servo and the radio tray. Then we discovered that it was overtightened. And the linkage between the Servo and Sevo-saver was not quite flat so we spaced that out a little and it seems to have solved the proble. I will be keeping a close eye on itfrom now on though to see if the problem resurfaces.