Many thanks for that Howard.

I read doc and switched to Native mode which opened up 3 radio buttons in the Dialin property: Allow access, deny access and control access through RAP (which was greyed out in Mixed mode).

However, all variations on $douser.put('MsNPAllowDialin',?) with ? being 0, 1, -1, true, 'true', '1' etc still give the same messagebox 3 error message with Control Access Through RAP being the option selected.