Clutch Fitting Primera 2.0 Diesel...HELP

I'm currently in a dispute with a garage who recently fitted a new clutch to my primera 94 diesel. Its come back with an oil leak to the near side drive shaft. The garage maintain it is not their fault, that they have not disturbed the seals in any way and that it must have been leaking before they did the work on it.

I was wondering whether it is possible for them to do the work on the car without disturbing the seals? i Feel that i am being taken for a ride.

All help and advice would be greatly appreciated.

Thanks in advance