I won't repeat the full details, they're in a different thread, but basically it failed on the speedo calibration. The Stack speedo uses an odd system to calibrate itself, that isn't adjustable/tweakable and doesn't (being American made, I guess) allow the margin of error that we have in the UK.
Essentially, first time round, it was reading only slightly over true road speed at 30/40 mph, till by the time you got to an indicated 70, you were actually doing a measured 70, and above this, the speedo said you were doing lower speeds than you actually were.
Because the speedo can only be calibrated by driving a fixed (two mile) distance in set up mode, I couldn't fix it there and then in the lane.
I shortened my calibration "course" to about 1.8 miles, which made the speedo indicate 70 mph for a true road speed of 65 mph (GPS measured) and on the re-test, this was sufficient "margin".