It doesn't slip off but it does move about 0.5cm away from the flush point of the belt if it were to sit perfectly.
Since my original post about the belt slipping, I have made a washer system to act as a lip on the drive pinion to stop this problem.
I have done about half a mile after fitting and all seems ok, but if the belt is slipping ever so slightly as it seems to be, eventually I think I'm going to wear out one side of the belt where it makes contact with the washer.
Since I have a new motor mount aswell I'm not sure it's a motor tilting problems, as all seems very straight. So not too sure what the culprit is as replaced all parts in drive section so far.
Regards
UPDATE:
So far after a few more miles the belt stays on as expected however there is very very slight signs of wear on the edge of the belt, but this shouldn't be much of a problem.
The only problem is hopefully resolving the original cause of the belts movement.