Leaking diff. 98 300tdi

Evening all.
Need some guidance please.
Going round a roundabout today there was a clonk from the front end. Sounded like a bit off road debris being flicked up and nothing untoward happened so I carried on regardless. This evening however I've discovered a puddle of oil under the front diff. Further investigation uncovered a pinhole leak right at the bottom of the diff with a small bulge of metal in front of it. A couple of things come to mind -
1) Can this have been caused by something amiss in the diff that has whacked the housing, causing the bulge from the inside and so the leak? Frankly I don't know of the diff housing is cast alloy or cast iron. Everything has continued to work as usual (I've only had her since May, please forgive my ignorance). This doesn't feel likely, I hope I'm right.
2) Is the bulge a patch of chemical metal that was used to stop a leak by a previous owner, which has failed in some way.
Assuming the second option I've applied some more chem-metal and checked the oil level in the diff - still plenty, but I can top it up in the morning. Other questions come to mind.
The compound I've used is rated to 160C - how hot can the diff get?
The plugs are taper fit and so can be expected to be tight. Does the oil get pressurised in use - is my patch likely to blow off.
Any comment or advice gratefully received, I'm supposed to be driving up to Leicester next weekend from Portsmouth and would like to feel confident..

Cheers
Turk

part of your diff has broken most likely the cross pin, remove diff and check and once out hole can be welded up

The oil does get hot, but don't know how hot it gets. The axle is vented though, so the pressure will not be a problem. If you look along your axle you will see a plastic pipe, about 5-6mm diameter, attached to it. This pipe goes up through the engine bay and is held up against the bulkhead, to prevent water ingress. You just need to make sure it's not blocked, kinked etc. You could visually check it or blow in the end if your agile enough to reach it
I see you're relatively new to the forum so you may not yet be aware that James' diagnosis is almost certainly correct.

Thank you both. I had a feeling something was up, the clonk was quite significant.