I live in a central London block of flats with a rooftop, communal TV aerial. Until yesterday afternoon Freeview reception was excellent. But since then I've been unable to receive the Multiplex 1 channels (BBC 1, 2, 3 & News 24), though all other channels are fine. It's not my Freeview box (I have two, and both have the same problem), and it's not the transmitter (another TV with indoor aerial is OK); in any case, a neighbour has the same problem. So I think it must be the communal aerial, but can't think what could have suddenly gone wrong. Any ideas?