Ensure you have the round plug in on LH top, re check all plugs are fitted tight...
Pretty sure the round plug was seated properly. I removed it and re-inserted just in-case, but fairly sure that wasn't the problem.
The 4 pin square plug above the small sump - hmm - didn't look quite right so I unplugged and re-seated it.
Whilst I was there I might as well unplug and reseat the selector switch connector too.
Didn't bother with the speedo sender - that was working.
Started the car, and undid the GB filler plug. managed to get about 1L more in before it started coming out again. Total now about 8L, so perhaps another check in a few miles time.
Anyhow, went for a test drive - car now behaving properly, and no MID Gearbox warning. So, a tentative success so far, though I'm not signing it off as done just yet. Need another 280K miles on the clock before I'll accept the problem as fixed