Oil leak from rear diff

[tobesvw]

My 1999 A6 2.5 tdi recently passed its MOT (phew!) but had an advisory that there was oil leaking from the rear diff. Apparently the area round it is 'damp' but oil isn't gushing out. Does anyone know how serious this is or what I need to do to sort it? Ideally don't want to spend money if I can avoid it!

Any help appreciated!

Thanks