Thank you for your replies.
Tacho calibration: Was done a few years ago when I upgraded the alternator. At that time max rpm was just about 3000, so I'll check if that's still the same. I'll check David's other speed/rpm thoughts at the same time.
The 2-bladed prop is clean and appears fair. It was professionally repaired, faired, and balanced about 3 or 4 years ago after it hit something, so I think it's alright.
The only wind was the wind due to my motion, and current there was a little, possibly half a knot, which may explain the difference between GPS and knotmeter.
And the oil was clean. Not sure if I'm pleased or not - if it was black that would probably give me the answer, expensively, but being clean leaves me still in doubt.
I will have a go at it this weekend. I'll run across the wind in a current-free area and check knotmeter, gps, and tacho in both directions. See what that turns up and let you know.
Thanks
John Gardner