After a lot of you tubing I bit the bullet and cleaned it all up. Wasn't as bad as it looked and the actual rust was very superficial. Just need to apply rust convertor, prime and paint, its out of the way enough that I can tolerate any paint mismatch.
However back onto the seal, has anyone a picture of what the seal should look like? Investigating more suggests it's been glued back in place at some point which I'm guessing wasnt a factory fix.