Rear Differential advice

I have a 2008 Patriot Ltd 4wd CRD (Diesel) here in the UK, its just done 90,000 miles.

As a precaution I just had the front diff, manual gearbox and rear diff fluids changed. (Local garage)
Unfortunately I have started to notice a whine from the rear diff, started about 2 weeks after fluid changes.
Looking under the vehicle I can see what looks like oil dripping from the drain hole in the rear diff.
I strongly suspect that its gradualy been losing oil until its now quite noisy.

It's going back into the garage on Friday to be checked.

What would anybody suggest I do now?
Assuming they drain the fluid, check for bits of metal and see how much comes out - what next?
Putting fresh oil in and fixing the leaky drain plug is only part of the problem.
My concern is as to how much damage may have been done and how prematurely the rear diff may now need replacing due to its life being significantly reduced.

Thoughts, advice and suggestions welcomed.
I plan on keeping this car for another 3-5 years and cannto afford to have to replace the rear diff if it fails early.