I've got some Crosso panniers that have got quite a few holes in. I once fixed a drybag backpack with patches of leather and industrial strength glue which worked like a treat, but I've forgotten what glue it was (it came in a large tin and I spread it on with a knife)- it was in the textiles department at the uni I was studying in. Any advice / links / tricks would be great.
You could sew your own patches on using canvas and high-strength thread. They even sell kevlar thread that's useful for this type of thing.
If it were my panniers and I prefer strong-but-ugly, then I would cut up an old inner tube and use those as my patch materials. There's a very strong contact cement called Pliobond that you can use to glue pretty much any flexible material to another one.
You can even make your patches from leather (still using Pliobond) or canvas (ditto).
Or you can lame out and go to your local outdoor store that sells a broad variety of patch materials for backpacks and tents: http://www.rei.com/c/gear-repair
If you have water proof panniers made of soft plastic, patches made for fixing holes in bike tubes are really great (depending on the size of the holes in the panniers of course).