My 8-year old 28"Avant 100Hz DVD/STB/RF/DSS now only shows a dark screen (though the opening of the electronic curtain is just visible) when I choose SAT. Yet it functioned well until a few days ago and TV and DVD continue to function as well as ever.

I thought the trouble might be the STB, so I bought a new one, but it made no difference (though it was not listed in the Sources) so I went back to my Panasonic TU-CT20 (which is listed and remains selected).

I tried an 8-hour period without power, then switched on, but the fault continues.

What happens if you swap scart sockets? I presume your DVD player is plugged into a scart socket, so what happens if you connect the DVD to the socket where the STB was and vice-versa?

Also, are did you try the new STB with the original scart cable? It may be down to this being faulty.