You're describing exactly what happens, Snibble, and the problem has, after all, come on in Autumn. I was interested in John's ideas about air getting to the prop via the swim, rather than under the counter but DecaB is probably right - we haven't lightened the boat enough to cause that. The boat reputedly has a draught of only 2' and I haven't noticed other boaters with the same problem, which was why I was looking for another answer.
We're cruising on through Fradley to Great Haywood this weekend so I'll take special notice.
SteveE