I don't think I've ever used .validate.nulltest for this. Not sure what the difference is (except maybe the exclusion groups!).
For all field types I use:
field.mandatory = "error";
field.mandatory = "disabled";
Thanks for your response. It seems the code was ok the problem was that the behaviour of the exclusion groups is different depending on Version and wheter the form is static or dynamic.
Again thanks for your response