Thanks High Wing Drifter - that AIC is I think the source of the confusion, and I'm still confused. It states:
Airworthiness notice AN84 describes the airworthiness requirements for VHF communications and ILS/VOR navigation receivers... It should be noted that this AN requires all equipment not meeting the new FM immunity standards to be identified as so with a placard, and their use restricted to VFR flights only.
There's nothing in the ANO to support 'VFR flights only'. You only need specified equipment to fly IFR in CAS; outside CAS you simply need whatever equipment is necessary for you to fly the intended route. That route could easily be a mixture of NDB tracking and dead reckoning, in which case you don't need FM immune VOR/ILS equipment.
It could be referring to the COM radios... but in AN84 (thanks for finding that Justiciar) it says:
Recognising the reduced risk (of interference with VHF communications), paragraph 5 of this Notice permits a relaxation from the ICAO standard for VHF communication receivers carried in aircraft of 5700kg MTWA or less.
Tim