Since fitting volcano block 24hrs ago been unable to get the fan above 25%. Just fitted a single sheet of Kapton film, with a hole cut for the nozzle and two "legs" which are held with crocodile clips to the wiring of the printhead. Took the fan rapidly up to 70% with no difficulties. Could have gone further but got bored coaxing buggy Marlin (that resets the nozzle temperature downwards each time you set the fan in the control menu because the Tune menu doesn't work.... Sigh.). Strong sense that this is both the necessary and the sufficient solution. btw. Kapton film is perhaps more rare than Kapton tape - and certainly more expensive - but I happened to have some lying around as a result of an aborted attempt to make solder mask stencils with a lasercutter. It's advantage here is it's just a bit stiffer and so makes a nice flexible plane that seems to holditself in place when attached only at one end.