Ok, it was indeed incompatible RAM.  I had bought HyperX 16GB DDR3L CL9 (2 x 8GB).  This ran at 9,9,9,28 and caused a problem with anything in slot #1 or both together.
I now have Corsair 16GB DDR3L CL11 (2 x 8GB) and it works fine.  This is on the tested list, part no. CMSO16GX3M2C1600C11 and runs at 11,11,11,28.  The full 16GB works in dual channel mode.
Sorted!