Thanks Bushfiva.
I ran the Lenovo Solution Center comprehensive test option today. System froze during the random number sequence test on the RAM.
Memtest86 includes this test and it passed several times when run from a bootable CD.
Can I assume that the memtest test and the Lenovo one are the same? I suspect that the hidden gremlin brought things to a halt, as the machine was booted into Windows (10).