I read the smb.conf and couldn't see anything wrong with my simple smb.conf file. However I decided to restrict all of the user access to the 'nobody' account.
To do this I changed the security from 'user' to 'share', opened up the public share to be writeable by setting the 'read only' parameter to 'no', and set the 'guest only' parameter to yes. I also changed the ownership of all the files in the 'public' share to 'nobody'.
I don't knowingly use Kerberos, so I don't think that has any bearing on the issue.
I put in a 'valid users' parameter, but according to the man page for smb.conf, this should be a list of users rather than an xp address, so I set it to 'nobody'.
My smb.conf file now looks like this:
workgroup = WWG
netbios name = SRV1
security = SHARE
valid users = nobody
path = /export
read only = No
guest only = Yes
guest ok = Yes
However, now that the share is writeable, the behaviour of files is more ionteresting. I can create files in XP, but not rename them. I can copy files to the share, but then cannot read them. Having created a file under XP, I can rename it on the Linux box, and amend the contents. I can then read the amended file on the XP box, and can copy it elsewhere, but I can't rename it under XP or do any further editing on XP.
Very strange. I'll keep trying.