[rsk289]

Thanks guys - the effort's actually slightly easier as the rods and levers are now working at 90°, which is more efficient.
The rods don't slide - the slot is simply for adjustment, there's a nut on the back of the pivot which you can't see which holds it tight in the slot. I was careful to weld with the adjustment in the centre of the slot just in case.
There's a small hole at the top of the levers, just under the shifter pivot, visible in the first two pics. This goes right through everything - put a 3/16" drill bit right through all the arms, set the side levers in neutral, tighten the slot nuts just mentioned and the shifter mechanism is adjusted correctly.
These things are so simple, it's unbelievable!