What was wrong with the engine when we went out in the river?
On closer inspection, it looks like the exhaust pipe is restricting the drive lever. There isn’t really much space and Steve decides to cut a notch in the metal frame the engine rests on. Like much of the boat, it’s over-engineered, so, with lots of sparks and noise he cuts it away making space for the pipes and good clearance for the drive lever.
It will need another test out in the river, but experiments driving forward and back in the berth put our minds at rest it’s neither the engine itself nor gearbox that is the problem.