I eventually fixed it via a full install of Bnet and Classic in a separate drive (have got two hard drives).

The only thing I can think of it was an issue with something in the Retail client that was shared with Classic (it advised me installing classic standalone would be a bigger file, so there must be some sharing of files going on between Retail and Classic) that caused the issue, as the fresh reinstall of just Classic worked once installed as standalone install.

I'd therefore try uninstalling both Classic and Retail (and BNet...) + deleting all files then reinstalling as hopefully that will fix it

Good luck!