Are you talking about Freeview reception in total, or trying to get the RF2 out signal from the Sky box?
Alexandra Palace muxes - 31, 32, 37
Crystal Palace muxes - 22, 23, 25, 26, 28, 30
Crystal Palace local mux - 35
When you say you get nothing from ch 50-68, if we're talking Freeview then...