A bit more conclusive this time, I was very careful to warm up the CT thoroughly, with 15 mins of effort from 100w through to 5 mins at 260w. Tire pressure was 110psi and cadence was maintained at around 94 and speed around 25mph.
As you can see the SRM and CT tracked very closely indeed - the SRM gave average power for the hour at 221w whilst the CT had 217w. Thats pretty darned close and drivetrain power loss can account for the difference.
Trouble is the calibration I performed last time was 20 mins at 160w and should have been good enough. I'll be using the SRM from now on to capture power data whilst on the CT since it is not so sensitive to calibration.
If you want them the data files are here.