Soun'ds like a bypass to me.
About 4" long, 1" high, 1" thick black plastic cover with gold lettering
and in/out cable sockets?
I still have one in the loft somewhere.
Given that they have not been supplied for years the CS may be new & not aware of them.
What sort of problems are you having?
Can you check the braiding around your aerial cable, it may be frayed, you may even have water in it.
If you can unscrew the bypass from your STB & aerial cables, flip the cover off & look at it, if it's all covered in white powder etc (from damp etc) then give it a good clean with an old toothbrush (or er indoors one) and reassemble, that sometimes cleared up problems I had.
Your unlikely to get it replaced by CS as they won't have any, however an engineer's van might just have an odd one kicking about, you might have some luck stopping one in the street & offering him a brew & biscuits in exchange for one
