I duplicated the site on a test area, copied everything across including the dbase and ran the upgrade, still had the usual errors, etc, but its up and running now after performing many of the steps above, but the front end skin was still a mess, footers and headers pretty much all over the place and looking very untidy. Imported another theme, basic, and everything is looking a great deal better although I am still having issues logging in as a customer. If I use the "sign in" link at the top of the page, it doesn't work, but If I browse the site and add something to the basket, THEN proceed to checkout I'm able to log in as a customer, but browsing any further just logs me out again, including trying to get back into the order screen just after ordering!. Uninstalling and re-installing the "sign-in" module makes no difference, other than to completely remove the module from all positions which means we need to add it back into nav2!.
I read somewhere it could be to do with the password column changing from 32 to 60 and by importing the dbase it's screwed up but can't figure out how to fix that.
All in all, I'd say we're (hopefully) on the verge of completion once that issue is dealt with so if anyone can advice I'd appreciate it 
Cheers
Marko
NB - spoke too soon - now getting "unable to update settings" when trying to edit a product, another bit of searching around shows people were having that issue back in 2017, presumably from upgrading too, will continue to try figure.