Hiya,
sadly all of these senders work in the 2.4Ghz bandwidth, the same as ALL wireless networking gear, and quite a few phones, oh and also microwaves (they creep into this range).
I've just tried two different types of sender and both such, i do live in the middle of london with lots of wireless noise, which is probably the main problem - also 2.4Ghz doesnt 'do' corners, it either passes straight through or bounces, thats why wireless tends to be bad in old houses.
You can get 5.8Ghz (i think) devices from the states, they will probably be brilliant...BUT they are illegal in the UK because that licence is not for public usage.