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!