So YZ250 popped over this morning, and got the switch out in around 20 minutes. Mrs TB and I were then cursing he'd done in 20 minutes what we've tried for for 4 hours on Friday and 7 hours on Saturday

We then tried to work out the reset mechanism for the switch, as there seemed to be zero information online. Both of us messed around with it for a fair while until we sussed it.
Then YZ250 refitted the switch, which took nearly 3 hours of fiddling to get it lined up blind, and at the right angle, and without "firing" the latched reset mechanism.
Quick drag check now shows both switches in the brake switch working as they should.
Massive, massive thanks to YZ250 for popping over - he's not exactly just around the corner - and for his perseverance on it

I hope you don't hurt like I do, it's not a comfortable position to access it the way you did, laying with your back on the cill for ages.