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@Dammit
Right, I took the long way home, the following is what I did, what I observed and my conclusions.
Before getting on the bike I ran the calibration, which was 507 and the torque was zero.
I set off and did some laps of Regents Park, power was what I expected to see.
Then just sort of randomly wandered South ish, looking for a combination of stop/start and longer uninterrupted roads.
After a few periods of track standing the power figure crept up- until I was putting out 400 watts just pootling along.
I stopped and re-calibrated- and as I did so noticed that with no force applied the torque was reading 9.98.
Off I set again, and again the power "looked right" at first, but then crept up again.
I rode for a longer period this time, then stopped and re-calibrated- torque at rest was 13.48 this time.
Once calibration was run power values were (once more) back to where I expected to see them.
I think that what is happening is the Garmin 910XT is performing an auto-zero when it thinks I am coasting - but of course with a fixed drivetrain you never truly coast.
I need to disable the auto-zero, and I cannot for the life of me find that option.