Managed to get it working - HDD formatted to MacOS Extended (journaled). I think the key was not to select "Backup Now" rather than wait the 120 second countdown for Time Machine to do the first...

I can see the External HDD in Finder and I can even select "BackupTM" as Time Machine backup disk (after entering username and password). The it waits 120 seconds to backup and (as per original...

Hi Tony Are you saying that TP-Link doesn't work at Time Machine Backup in Access Point mode? One of its two standard modes?! Very disappointing if true.