I would really appreciate knowing whether this person is talking utter nonsense - or has anyone actually heard of such a thing as a Restricted Unrestricted FI Rating?
It's utter nonsense. If you have met the experience requirements using another ICAO-compliant instructor certificate, then you can apply for an unrestricted EASA Flight Instructor certificate having completed the EASA FI conversion course and associated FI assessment of competence. See CAP 804 Section 4 Part Q Subpart 2, page 5, para 3.6.4:
3.6.4 Applicants who can demonstrate they have met the experience requirements for the removal of restriction of FCL.910.FI on the basis of their ICAO Instructor rating may have the Part-FCL Instructor certificate issued without this restriction at initial issue.
http://www.caa.co.uk/cap804
ifitaint...