My indy did mine as I had exactly the same scenario with the belt going. He took some time to explain to me that you can't just pop the seal in, there's actually a special tool (can't remember what it was but it wasn't fancy) and if you don't install it correctly it will leak again.

I think the seal is actually leather or something similar ?

If you give any of the chaps at Bell Automotive a call (01244 813321) they'll explain on the phone.