Of course I spoke too soon.   With either an Apple 20W iPad (3A 5V) or a Belkin Boost Charge Pro this happens. I take the point on not being sure with consumer grade, but honestly, nothing else has this issue. And it could just be a dud, the fact no one else has piped up with similar stories is leading me in that direction.  Only other thing I can think is when I soldered on the GPIO header I shorted a trace, but I don't have a soldering iron with me at the moment so reflowing will have to wait.   thanks