The measured resistance from the white multiway you have shown to the C30/31 earth studs is 0.1ohm, this is identical to a measurement between two scratch marks on opposite sides of the boot floor, so assume that is OK.
As I sit here I have noticed a double sided nail file board and wonder should I se that to roughen the contacts surface?
Well on Sunday after going through all of the checks previously mentioned and dismantling the BC and spraying the sockets with contact cleaner the damn warning triangle was still there, so I gave it a rest.
Come Tuesday evening I started up the car to go out for the weekly pub visit and 'Hey Presto' no warning triangle! I have no idea what has caused it, I am just please it has disappeared.
Thanks again to Deckchair and Davren for their advice - until the next time