OK, thanks for your response. I will investigate it further and if necessary, I will have to rebuild the view using the new blocks approach which I use on other sites.
This was difficult to solve and has taken an hour and half, but now resolved.
It seems that there was some issue with the WordPress user permissions for Administrator Level. Some customisation of the roles and permissions in the past must have upset something.
Even Toolset Access wouldn't allow me to change the permissions for Administrator level (just hung on the spinner).
In the end changing the General Toolset option to "Show both the legacy and Blocks interface and let me choose which to use for each item I build" seemed to resolve things.
After setting this, suddenly I could save the changes to the View again (and solve the original issue which led to this bigger one!).
Thought I would let you know in case it helps you to assist others. Most of the ideas which led me to the solution originated from other Toolset Support tickets. Got there in the end!
And now I know all about Toolset Access which I hadn't used before, so a bonus.
