Davey wrote:

My gut feeling would be to remove all the salt that's in the main
compartment, and then have a good look for a crack or leak somewhere in
the working body of the softener.

Thanks for that. Removing the incursive salt is inevitably going to mean
that some of it will drop into the low-level water in the main compartment:
I can't see a way of avoiding that, but perhaps it will do no harm.