I braved it this afternoon and decided to remove the cover off the synscan mount. There is a nut behind the part that holds the scope inside the case which has two washers. The nut was loose so I tightened this slightly and tried it. Didn't work well first time so undid the nut and removed the mechanism and cleaned around the cog connected to the motor and there seem to a build up 'space dust'. Tried it all again and it seemed to move up and down nicely. Put the whole thing back together and fitted the scope back on and tried again, and all seems good, nicely moving up and down and no slippage of the scope. Fingers crossed that I've solved issue.
Thanks everyone for your help!