The oli Sparrow solution suggested by tigat looks as if it's vapourware - I think it was a design competition gone big and never made more than a few. A shame but I reckon it isn't exactly commercial even though I would have been happy to pay quite a bit for it.
Wasn't surprised to hear that the 'fixed split ratio' solutions have serious drawbacks (however you do it) but still great to hear from those who've tried some of the solutions i've considered (including knee-operated & wackier. I'd hoped that with the Paralympics there may have been some really innovative engineering.
As a kid I had a back pedal brake but that doesn't go with gears so for years I've effectively had the one brake.
Seems to me that the best way (using readily-available parts) is probably the 'two lever' solution. I would like to see there is some way I could modify them so that they sit next to each other naturally. I'm going to experiment.
Anti-lock brakes? I guess it must be possible and maybe the next big thing in bike brakes. But a way off I think. And rather longer than that to be accepted by the regulators.
Thanks everyone
Jeff