Update:
I was asked to downgrade the firmware on my UM2 to 14.12.1 which I did. It seemed to fix the problem and I didn't see any problems on subsequent prints.
However, during a print, I acknowledged a sound that had been happening since I first started using the UM2 (I just hadn't got around to checking it out). It was a knocking/clicking sound, as if the print head was hitting something. On investigation it turned out to be the bed lead-screw (threaded rod) turning slightly, then releasing (this occurred mid layer, it wasn't moving to a new layer). Once I had found this was making the noise, I decided to place a light rotational force on the lead-screw with my fingers. As expected, the bed stayed still due to the holding torque of the motor, however, when the noise occurred, it released the holding torque and the bed lowered!!! I expect this was the problem all along, and that firmware version 14.12.1 has somehow improved the issue, but not resolved it. If anyone has seen anything like this, please let me know.