No, it wasn't on a lock cill; it was as I was pulling in to drop Jim off at the innocuous and unassuming looking Branston lock, I just very gently reversed onto a rock about three feet out from the sheet piling, and knew immediately what had happened, despite never having experienced it before.
We struggled with it for a bit and managed to get it sitting (as we realised later) on the skeg but not in the cup. It didn't feel right but we struggled on, until it fell off again, catching Jim's hand in the process, as the tiller bar dropped a couple of inches, against the sharp corner of the rear door hasp (important job: grind that sharp corner off).
So we limped on very slowly until we got to Shobnall Fields in Burton, and having fortified Jim with lunch and a cup of tea and a great deal of sticking plaster (and TCP and a rubber glove), resumed trying to resite the rudder in its proper place. The only way really to do it, in the water, is by trial and error, Jim lifting the tiller with his shoulder. Every time we thought it was there though, a few trial sweeps led to it dropping out again. Eventually it was Iain who, employing the same method, either by luck or experience achieved success.
So on we have gone, ending up eventually just beyond Swarkestone lock, where we have not been to a pub but have enjoyed a lovely meal with Clair and Iain on Plover.
The starter motor is performing excellently, and today we have tested it on starting the engine when it's hot, which it has achieved without a murmur.