So, the box was stripped today. Pretty typical Triumph gearbox maladies I'm afraid. The front needle rollers on the layshaft had failed; I'm pretty sure the rollers are supposed to be round, not square 😳
The rear rollers had also started to eat into the shaft, it at least those were round.
The layshaft is junk, it had to be cut in two to remove it without causing damage to the case, and worse the inner part of the lay cluster where the rollers run is damaged.
Looking at the price and availability of the lay cluster, I'm going to investigate having a sleeve fitted, but I won't know if that's viable until I've visited the local machine shop tomorrow.
On the up side, everything else on the box and o/d is very good, but then you'd expect that from a gearbox which was overhauled 3500 miles ago.