So, just gone round with my new Michelin tyre pump (allegedly accurate to within + or - 1psi) and set all wheels to the exact recommended pressures and recalibrated the system again.
The light coming on at high speeds may, or may not, be a red herring but, interestingly, there is a whole thread on Honest John about the light coming on in a CRV after driving at 70 mph.
If it comes on again, I'll take it in to the dealers. False alarms defeat the whole object of the system.