Some years ago a friend had a leather bag repaired here:
Go to Kallis shoe repairs in Crouch End. They virtually repair anything leather - I've taken them all sorts of stuff to fix. Friendly, excellent service and they know their stuff!!
I tried them, it's beyond their powers.
Ahh, if they can't fix it, maybe time to get a knew one then! Good luck x
Try Modella shoe repair shop near Turnpike Lane tube. They fixed the zip on a musical instrument case for me recently, when every other local repair shop, drycleaner etc said they couldn't do it.
That sounds promising, thanks!