Rick emailed me to say my sums are wrong (see post below). 10.7 million gallons would weigh 47000 tons!! Rick is usually right on these things and I automatically defer without checking.
That's a hell of a lot of water to go missing. Enough to float the QEII in fact. It must be true, Wikipedia says so :-) Now we'll never see the QEII on the Grand Union.
I'm pretty sure the level has dropped because all the way along the canal you can see the damp tide mark of the previous level. I blame the Met Office. The weather has been worse since they left Bracknell and moved to Exeter.