I was just about to re-run the whole installation, with TOS 4x, when the 'Loading' screen disappeared and I got the TOS 5 desktop (in Vivaldi) - it took about 40 minutes. Wonder of wonders! There was some EULA (nonsense), which I couldn't get rid of as it wouldn't let me 'confirm' - until I realised there was a scroll option. I clicked & ticked and it all seems fine, so I'm not going to struggle with the down-grade at the moment. So far, so good! (I'm going to manage without TNAS PC.)
The only problem I've got is that the volume size shows as less than it should: this is because I upgraded the HD from 2 x 2TB to 2 x 4TB in RAID 1 recently. (Volume was wrong in TOS 4 as well.) I found this thread: viewtopic.php?f=75&t=1543 (Online RAID Expansion), but I'm hoping there's a way to change the volume without removing the drives, etc.
Anyway, I changed the IP back and the NAS is working properly now. Thanks for your help TMroy.
Cheers,
Chris