In order to be able to manage OSX Macs with workgroup manager, the client Macs need to know where to fing the LDAP address of the server. This is usually set on the DHCP server using the LDAP Server setting.
That all sounds great, but our DHCP Server is a windows 2003 Server and the LDAP Server option (option 95) seems to have mysteriously been left out. It seems there was even an attempt to remove it from the DHCP standard "::because no-one was using it":: linkname but Apple pointed out that thay had been using it for the last 4 years.
Buried deep in the microsoft support pages I eventually found this which lead me to contrive the command:
netsh dhcp server add optiondef 95 ldap-server string
which will then give the option to add an LDAP Server to the DHCP options being sent out. Now all the Macs are happy.