i have a couple of points toadd, based on experience.

1) the crank pulley should be lined up so that the leading edge of the square peg is in line with the blade of the timing marker, rather than the centre of it as pictured (according to the YB service manual).

2) tensioner. the manual says to tension the belt by turning the tensioner clockwise and locking it. if you do this, the belt is trying to turn the tensioner anticlockwise which has a loosening action against the locknut. i had this happen that the locknut loosened and the cambelt loosened and jumped and fucked all 16 valves.

if you tension the belt by rotating the tensioner anti-clockwise, the belt pushes against it and keeps it tight against the locknut.