Airworthiness Notice 84 states:
"5.2 Requirements for ILS localiser and VOR receivers effective from 1 January 2001:
(a) ILS localiser and VOR receivers, required to be carried.by UK registered aircraft for the purposes of operations under Instrument Flight Rules (IFR) in accordance with applicable airworthiness and operational regulations, must be of a type approved as complying with the improved FM broadcast immunity standard.
(b) Where non-immune ILS localiser and VOR receivers remain installed, (i.e. those carried in addition to the minimum number required by applicable airworthiness and operational regulations to meet IFR), they must be identified to the flight crew and their use restricted to Visual Flight Rule (VFR) operations.
NOTE: For aircraft of 5700 kg MTWA or less, an acceptable means of compliance is to placard and restrict non-immune receivers to operations permitted under the Restricted Approval Category LA Class 3 (see paragraph 5.4), irrespective of the approval category for that equipment.
5.3 Requirements for VHF communications receivers effective from 1 January 2001:
(a) VHF communications receivers, required to be carried by UK registered aircraft over 5700 kg MTWA for the purposes of operations under IFR in accordance with applicable airworthiness and operational regulations, must be of a type approved as complying with the improved FM broadcast immunity standard.
(b) Where non-immune VHF communications receivers remain installed in UK registered aircraft over 5700 kg MTWA, (i.e. those carried in addition to the minimum number required by applicable airworthiness and operational regulations to meet IFR), they must be identified so as to alert flight crews to the potential risk of interference.
(c) UK registered aircraft of 5700 kg MTWA or less may continue to be operated under IFR with non-immune VHF communications receivers provided that the receivers are identified so as to alert flight crews to the potential risk of interference.
NOTE: CAA will continue to monitor reports of interference and, if necessary in the light of experience, reconsider this relaxation from the ICAO standard for VHF communication receivers carried by aircraft of 5700kg MTWA or less."
Basically this means that if you're using a VOR or ILS
anywhere in the UK in a light ac, unless you're under VFR, you MUST have a FM-immune Nav receiver. However, your light aircraft Com transceiver doesn't need to be FM immune.
I went through this whole absurd nonsense a couple of years ago and it cost £17k to upgrade all my 4 ac to FM-immunity compliance requirements.....believe me, if it hadn't been essential I wouldn't have spent that amount of money unnecessarily!