Hello Dave,
How natting was done at your end? and also one spare public ip.
follow following steps.
eg.
public ip : 1.1.1.1
inside ip : 192.168.1.1
1. access-list ouside_access_in permit tcp any host 192.168.1.1 eq 3389
2. object network inside_rdp
host 192.168.1.1
nat (inside,outside) static 1.1.1.1 service tcp 3389 3389
3. access-group ouside_access_in in interface outside
jeevak,