Thank you Fotis,
I appreciate it is difficult trying to identify what the cause might be on different systems and setup.
I have checked the permissions and they are OK; the ACL is set to superuser with complete access; and I can access all articles and add/delete/edit without issues (so database is fine).
It's only when I go to K2->settings and then press Save, that I get the 403 message (even if I press nothing else other than Save). Very curious.
You mentioned in an earlier post that there may be a conflict with another extension - are there any obvious ones that you know of that could be causing this? Or perhaps something else I can look into?

Efcaristo.

Just wondering if plugin ordering may be an issue? Perhaps 'System - K2' needs to be run before certain other plugins?
Just a thought.

Thank you for the suggestion. Unfortunately even with system-k2 as the first system plugin, the 403 persists.
I even reinstalled K2 and no effect.
Looks like I'm going to have to move my 1000+ articles out of K2 and into Joomla.
Thanks

I'd prefer to get K2 working. Is there anything more drastic I can do? For example delete certain K2 files and then reinstall K2 so that the old files are written-over or recreated?

Yes, new site with K2 and transfer of dbase looks OK, and saving K2 settings in admin works.
Only thing I could not do was recreate the media links (no images with articles) but for this exercise, all good.
So this means...? extension conflict?