It is still raining and floods are worse than they were 3 weeks ago, so I have not yet returned to see if it is dry and I can get a check measurement between 7k and 8k to determine the effect of the flood on the calibration constant.
Pete asks about the the effects of hydrostatic pressure squeezing the tyre. We have the pressure coefficient for a tyre reported by Neville here. His figures suggest about 0.2% for 1 bar pressure change, at least for his tyre. But 1 foot of water would have only given me 0.03bar pressure on the tyre under the water. So it looks to me that the hydrostatic pressure effect should be negligible.