Empty bank balance!!
Talking from memory (so am not going to be quoting verbatim rules here - that is your longer term challenge!), it's now a requirement for any aircraft flying IFR in UK Airspace to be fitted with radio equipment that does not interfere with the performance of the VOR / DME, commercial radio, etc. FM Immune equipment is certified not to have any spurious artefacts in its transmission spectrum (or radiating from the box itself) outside the permissible levels. FM immune equipment is also now at 8.33kHz channel spacing.
Due to my lack of ability to provide a detailed explanation, I just found the attached overview:
FM IMMUNITY (CAA AIRWORTHINESS NOTICE 84)
WHAT DOES IT MEAN?
* Aircraft navigation and communication receivers in most current aircraft were designed to eliminate commercial FM radio interference. The design specification is now 40 years old and things have moved on. The restrictions under which commercial radio stations have operated for the last 30 or 40 years will be lifted on 1st January 2001. This means that they will be able to broadcast within an increased bandwidth and with increased power causing breakthrough (FM interference) in most navigation receivers in use today. This FM interference may cause loss of sensitivity and / or erroneous navigation information.
HOW WILL IT AFFECT MY AIRCRAFT?
* If your aircraft is used for IFR or IMC flying then you must take action before 1st January 2001. This means making your navigation receivers FM immune. Old analogue avionics such as Bendix/King KX170 & 175 series and Narco Nav211 series etc. cannot be modified and must simply be replaced. More modern sets such as Bendix/King KX155/165 series and Narco MK12D may be modified or service exchanged. Please note however that not all modern sets can be modified and often the cost of a modification is greater than a new replacement.
WHAT IF I NEVER FLY IFR OR IMC?
* If you only fly under VFR conditions you may placard your equipment to say that it does not comply with FM immunity and down grade it to class 3.
* Please read carefully the terms and conditions of class 3 equipment. If you fly with non FM immune equipment after 1st January 2001 in class A, B, or C airspace or under IFR/IMC conditions you will be breaking the law.