Surprised me as well. In the fault finding we had:
Checked pipes from water pump to thermostat for blockages
Checked exhaust for blockages
Replaced exhaust elbow
I was starting to run out of ideas, then inspected the impeller cover and it was very worn (from the rotation of the impeller spindle). Turned the impeller cover over so he worn side was now on the outside and no more overheating, much less steam, and greater water flow from the exhaust.
I was really not expecting the solution to be as simple as that, shame it took 3 months of overheating before I found the solution.
Rib Tigga an Avon SR4 Rescue with Honda BF50
MMSI 235910073 Callsign T123046