OK, to turn a common question on it's head. Our client machines (xp pro) can't access shares via ip address, but can by server name.
\\192.168.0.200\share doesn't work
I have done a ping server to ensure it returns the correct ip address and it does, and connecting to the share via IP address works if you do it on the server.
I can access other shares on the server via IP from client machines, it is just that one share that is causing problems.