In order, this is what I did:
1) Went to the 'price' screen and pressed the 'two price' button twice (once to show the 'r', once to take it away again)
2). Pressed the 'SET' button and stepped through without changing the actual price
2) Reset the cumulative totals (held SET button for 5 seconds when showing the 'total KWh')
... and lo and behold, when plugged back in it started to register again.
The voltage display, however, showed 214V - which is way outside the range allowed for UK supply. So I stuck my multimeter in the same socket and it registered a much more reasonable 242V.
I conclude from this that the B&Q Energy Tracker is fundamentally broken as a device.
However, that will not stop me taking this third one back to B&Q and insisting on yet another replacement.
Hmmm - this is the sort of thing that BBC's Watchdog might be interested in. I'll drop them a line, if you do too then perhaps they might take it up.