I paid a visit to Stealth Racing. There was lateral force being applied by the tower body onto the gate. It was nothing to do with the cables. They filed off some material and checked the clearance with a feeler-gauge. I was impressed! They only charged for 1 1/3 hrs labour even though there was 2/3 of them working on it and it was longer than that, anyway. They really know about anything to do with gearboxes .....
David