OK, so what I did is this:
Left it alone for a day, then realised that it wasn’t doing anything at all, so I copied a large folder (some 130GB) to an external drive, and deleted that folder from drobo.
That seems to have triggered some sort of inner workings in the drobo. (relayout?)
After a day or so of drives grumbling away, all fell silent, and I transferred the folder back to drobo.
When all was silent I tried playing back a large HD video file that wouldn’t play before, and this time it played perfectly.
I’ve not had time to try anything else out, but it would seem that the problem could have been related to the drobo not doing the auto relayout… I’m just assuming here…