Answered my own question sort of.

Needed to apply delegation to the SPNs for the new SQLServiceAccount to the webServer computer object && the Application Pool service account.

Find all of this...

Thanks all.

Most grateful as ever!  Even if it's just for ideas / the offer of more eyeballs (I can't post out though) & confirmation that it's probably not bloody SQL!! ...

Oh yeah.  Duh!  Thank you. Totally forgot that as an option.  Long day already at it's not even 11.  🙂

Thank you all!  It's so helpful to get your ideas.  I work alone and sometimes you end up down in the rabbit holes of your own mind so it’s great...

Thank you guys, I forgot to get back to you.  I had all sorts of stuff going on unrelated to SQL - gpos, permissions, etc. But I appreciate the reply. ...