All of the XP machines are getting the correct WINS server address from the DHCP server. I have checked this and it's correct.
I have also tried deleted the wins.dat file from the samba server, but when samba rebuilds the file, it still puts the old ip addresses of the clients in there instead of the new ones. e.g. one of the client machines was assigned a new ip address from the DHCP server (old ip 192.168.1.193, new ip 192.168.1.10) but the entry that appears in wins.dat is sill 192.168.1.193, even though the address has changed on the client. There for all of the other clients try and access the machine using the old ip address.
It's very strange, as I have set this up many times before and normally works perfectly.