Out of courtesy and for accuracy should someone else with the same problem find this thread:
Despite appearances yesterday, I was wrong; the problem resurfaced today but has now been put to bed thanks to some Linux pointers I was given.
- Using WinSCP I opened /etc/samba/smb.conf and changed Security = User to Security = Share
Given that the NAS was in use for over a year before the problem manifested against the XP PC in question (and it didn't manifest at all against the other XP and W7 devices on the network) I have no idea what triggered the effect, but the important thing is that it really is fixed now!
What will be interesting is if it also fixes the lock that eventually happens (at some point from minutes to hours) when I stream music to my Acoustic Energy internet radio from a USB HDD piggybacked on to the NAS. If not, I now have the Samba manual as something to play with, all other sources of info having failed to crack this one!