Hi Dave,
Welcome along, mate.
This was always going to happen with HF ballasts in lighting gear, the actual frequency is about 20kHz, so it sounds like this might be a harmonic thing coming from the ballasts.
One other thing to be aware of, make sure that the interference is actually coming from the lights themselves.
Reason I say that is because the interference could be coming from something else, like a wireless router (mine totally knocks out the AM band here).
