From a strictly legal point of view you must carry the nav kit listed in Schedule 5 of the ANO if you wish to fly in regulated airspace. So, if you fly in airspace where a VOR/ILS is required then it must be FM immune (for public transport you need 2 FM immune boxes). Outside regulated airspace there is no legal requirement to carry an FM immune VOR/ILS so you can fly a PAR/SRA/NDB approach anywhere you like as long as you meet the requirements of Schedule 5.