Well, I will check after a few thousand km's how much has leaked out, by refilling it.
According to the garage to solve it the complete gearbox should be replaced / overhauled, but looking at the diagrams an option could be to change the diff bearings, These seem accessible from underneath the car with the gearbox in place. Too much play will likely be caused by worn bearings I think. The only thing is, if these are pressed in, removal won't be possible, but according to the diagram the bearings are held in place by a snap ring, which would make things easier.
I will also check if the correct oil seal has been mounted by the garage.