Ok, I think this one is sorted. It didn't like the syntax within the search filter. Current settings that are working, hopefully someone may find them useful ) :
Bind DN format
mydomain\:user
Base DN
DC=mydomain,DC=internal
Search filter
(&(cn=:user)(memberof=CN=bookingsystem,OU=Users,D C=mydomain,DC=internal))
Cheers for the help Griff!
Ed