Thanks for your comments.
After I posted I went for a ride to see if I could get the ABS to activate, again. On a slightly dusty road I managed to get the rear to pulse a few times by applying really heavy presure to the pedal. I'm not sure if the performance of the rear brake has changed because it has always been next to useless so it could probaly fail completely and still have minimal effect on stopping distance.
I applied as much pressure as I dared to the front which just confirmed that the stopping distance was a lot more than I would like. Given the poor deceleration, I'm not surprised that I didn't get the ABS to activate.
The caliper pistons are fine. This is no surprise because the the bike has only done 5000 miles and has been ridden rarely on salty roads. Haven't checked the slide pin yet.
As I said to roadrash83 , the dealer made things better, for a while, simply by running their normal diagnostic tests and then measuring brake performance on their rolling road. I don't think that would fix corrosion or wear problems at the brake caliper.
I take your point that normally the ABS is inactive until needed but what if the ABS unit is faulty, e.g. a sticky solenoid or a leaky valve? Could that result in reduced fluid pressure in the caliper? Is it possible that a faulty ECU or sensor results in the system being in a constant anti lock state, again applying only reduced pressure to the brakes? These are the sort of things that might be fixed, at least temporarily, by the diagnostic tests and associated system reset.