I've been having a look at David's lock over the weekend. Everything seemed in good order and nicely greased, but the barrel was stuck fast in the surround. After sitting overnight in WD40 I managed to get it apart and discovered that it was just corrosion on the first few mm of the barrel holding it place.
After cleaning, it was clear why - the rubber seal was perished and disintegrated so moisture could get in.
The seal is a flat-ish moulded thing which is not available as a spare part. The thought occurred that an o-ring might work to replace it, and the oil cooler pipe o-ring appears to be just the right size
It actually goes together quite well, but its a little stiff to turn so it might need a smaller o-ring.
Or, I might see if I can print a polyurethane replacement for the original seal.