I had a fasting BG test done recently so I decided to use this opportunity to do a very unscientific test of the SD. The lab result came back with 4.3, but my SD reading (taken just after the blood sample was taken) was 5.2, with the Freestyle reading 4.5. As I said, not exactly the most rigorous of tests, but it does add to my suspicions that the SD readings have been unusually elevated for a while.
Incidentally, I also had a HbA1c test, which came back at 32. I think it was 33 when I had it done a few months ago around the time I made significant changes to my diet and exercise, so not much change there.
Anyway, it has at least put my mind at rest a bit.