[julie_m]

The lighting circuit will have a separate breaker from the ring main, so there will be a longer length of wire -- and therefore a greater inductance -- between the lamp and the receiver when the lamp is inserted in a ceiling light fitting. This is suggesting that having the lamp electrically close to the set is important, which tends to corroborate the idea of it being due to the lamp having a class X capacitor across the supply.

I'm staying tuned for the next instalment!