Seen the full breakdown of results on ZwiftPower now and am a little happier with my performance. Yes, my average for the entire race is fairly rubbish, but then I'll admit on the second lap while I was still trying to keep up with Jeff most of the time, there were various periods when my focus wasn't really there (mainly when I was listening to the food discussions on Discord) and my watts just dropped accordingly.
But my 20 minute figure (which probably takes me from the start line to the top of Libby Hill on Lap 1) is my 4th highest wattage in a race yet and only the 6th time that my 95% figure has gone over 2.5 w/kg, so proving that I did push my absolute hardest while I was in that back marker group. No shame in coming in last if you gave it everything!
EDIT - and apparently I didn't come in last after all! Two other late finishers are now listed behind me (at almost 10 and 15 minutes back). However there was certainly no-one else on the course map as I finished, so I can only assume these were people who lost connection at some point, then got it back in order to finish.