Battery cut off

deepfat wrote:

David I will have a look but I suspect this too is made for the negative terminal, the clue I spotted was the N stamped on it

yep, sorry for the confusion, I was replying to roverlike's query about what it is...

And my solution to the radio problem would be to run a separate earth to it so that it retains its memory when the battery negative is isolated.

Always disconnect the earth first, no matter if it is a negative earth car or a positive earth car. This is the safe way to isolate the battery. The battery cut-off should be on the earth lead of the battery.