I'm looking for a source who can repair a broken fusee chain.

Dave,
You could always try Chris Abell, I think he's done work on these.

If you get no joy anywhere on your side of the pond, Email me and I'll see if I can help.
I've repaired a few of these for my own watches, plus I've maybe a dozen spare complete chains of various sizes - one of those even may bail you out.

John