Thanks for the responses. I did some searching in the forum last night and I don't think I found any real clues.
Sounds like the flashing O/D light is the CTL (Check Tranny Light). I suppose these codes are read from the same port but with a different reader.
The problem has not reappeared and it seems to be changing gears okay. I checked the fluid level too and it seems okay. A bit low but still in the correct range. I will top it up as it should be at the top of the line according to some other TSB.