when you push the drive shaft into the diff by hand is there a gap between the cv joint and the bearing? or is there a 10mm gap?
if there is no resistance i would imagine that the circlip is either broken or never been fitted.
when fitting a driveshaft into the diff, there is quite a bit of resistance (piece of wood and a big hammer to persuade the shaft into the diff.
it sounds like the driveshaft has the correct amount of sideward movement.
the driveshaft might just need chapped back into the diff, you can do this by slipping the gaiter off the cv joint to allow you to get a piece of wood on the edge of the cv joint to hammer it back into the diff.
its hard to explain, its always ten times easier if the car was in front of me

bring it to scotland and i will have a look

lol
Hope that helps and let me know how you get on
Ed