Back in dock, we brought the headsail down to check it, and released from tension, it all seemed to work perfectly. Tried raising the sail, thinking perhaps I'd somehow lost tension on the genoa halyard which had caused the tangle, and now the mechanism won't run up the furling bar more than a dozen feet.
The halyard seems to pull the sheave out of true alignment (it's quite slack, with a fair bit of movement) at which point it jams.
A very frustrating end to a fantastic four day's sailing. Especially as it threatens next weekend's sailing.