Multiple OD Masters and issues at Login

I want to access multiple OD masters on the same machines as I have some accounts on an old Xserve and others on a Mac Mini Server. The X serve is running 10.5.8 and is also my mail server so I really dont want to up grade it to 10.7 as it works all day every day with hardly a glitch in years of service.

Threr are 2 sets of clients, ones in classrooms for staff admin are 10.5.8 and those in a couple of labs 10.7.4. The ones in classrooms are bound to the X Serve and show only those users and I'm quite happy about that.

Staff wnted to acces the newer machines and so we have been trying to get the 10.7 machines to bind to the 10.5.8 server. In the directory utility or whatever it is called these days everything looks OK and the users appear on the login list, but when you attempt to login you get the terribly helpful unable to login there's been an error message.

Now hen I originally set up the 10.7 labs I had this error and I sorted it out by adding some DNS settings for the mini server and all was well. I tried this by adding the X serve to the DNS list on the client, but no joy.

Anybody got any ideas? Have I just missed anything obvious?

Thanks for any help it would be most appreciated.