The problem was caused by the PC that originally experienced the connection failure having an account with the same name as one on the NAS that I had completely forgotten about; after I deleted this NAS account and set the smb.conf file back to security = user and guest only = no (i.e the same as it was before I started troubleshooting) all was well. Every PC on the home network can now connect to the Public shares on the NAS.
I'm annoyed that I missed something so obvious, but have learned a heap about Samba that I wouldn't have learned otherwise so will chalk this up as a success.
Rather than recently "starting to happen" as I thought, it must have been a problem all along but how I missed it, I have no idea. I can only think that something else that connects to the NAS was "opening" the connection for me before I came along and clicked it. A bit like I remember about 15 years ago when I couldn't get a PC to connect to a Sun workstation unless the latter was PINGed once first. Anyway, I don't care; it's fixed and I now have something else to fix (plumbing!).