[rovertone]

Thanks for all the input folks.
With a cold engine if I turn on ignition and press demist the rad fan starts running and goes off when |I switch off demist. Likewise the same happens when I switch on Auto on Aircon and fan goes off when I switch to econ.

The aircon is working ok

I trawled thru the workshop manual and could only find brief reference to engine safe /limp mode, which suggested that it is triggered if either water temp/ ATF or engine oil excessively overheat but no content that I could find to say what status the engine takes.

The first time the problem happened the engine power just gradually faded, I throttled right back, noticed then that the engine temp had risen, say 4mm on needle above norm half way point. Throttling back must have lightened engine load and power picked back up, temp rose and power faded again, did this about 6 times till got over the top and everything went back to normal.

Water, as well as cooling the engine, as far as I can ascertain also cools ATF so it could have been excessive coolant temp or ATF or both which triggered limp mode which led me to ponder on 2nd speed of fan not working to cool the water to cool the ATF etc.
As I said earlier I noted that HGV's going up this first climb were crawling with hazards on from fairly near the bottom so perhaps they were used to the route and knew the load it would place on engine cooling and maybe I should have been a bit easier.
If my fan is working correctly on both speeds then unless radiator airflow is restricted it was just the severity of the workload. I am fairly certain that airflow is ok but will have a look tomorrow, the aircon condenser sitting in front of the rad never struck me as ideal arrangement anyway. Thanks again for all your input.