I should have spotted this much earlier, but on Sheet 21, the two Elevator Pulley Brackets are shown mounted upside down. Made to the plans (as I did!) the two elevator cables are able to fall off the pulleys when the cables are slack or loose. This is not a problem as long as the cables are kept under tension, but they must be double checked if you detach the cables for any reason.
The carbon brackets that envelope the pulleys should be mounted the other way up, in such a way as to prevent this problem.
Phil.