Update:
I'd still like to know how to "fix" this, either by repairing the array or mounting the filesystem based on the testdisk info.
But I have now located a spare drive and installed it, and I'm using the testdisk "list files" function and its "copy" capability to copy the files from the corrupted disks to the new disk. Thus far it's working pretty well as long as the files aren't themselves corrupt - the handful I've checked have been fine but see separate post to the list about automating wider checks.
I find testdisk can be a bit counterintuitive to use but it is a fantastic data recovery tool that everyone should be aware of (along with it's companion "photorec" program for recovering photos and other files from a corrupt disk, which I have used more than once to recover photos from "dead" disks for friends and family).
Mark