Previously, your video was helping to boost the signal. Now it isn't. It sounds like you have an aerial problem. As you have only lost one Mux it may be that you need a wideband aerial. Come ananlogue switch off all will be OK because the Muxes will be probably be brought closer together in Frequency terms).
To fix your problem, route the aerial cable through the VCR like you used to. You can still use SCART for recodrding an viewing the VCR, you're just using it as a second boooster.
Alternatively have your aerial looked at by some one who knows what they're doing.
Terry
November 2006