OK, I've found that the slot in the new layshaft I've got is not quite as deep as the original (0.170" new layshaft compared to 0.193" on the original layshaft). This small difference results in the locking plate not falling into the correct position to allow the bolt to pass through!.
You can just see the witness mark left by the old layshaft when the plate is as far in as it will go.
Before I start grinding a bit off the locking plate; have other people experienced this problem? Don't want to have missed something silly.












