rick wrote:
Have knocked up a fan speed controller ... where temp control speed ...
and hence cooling.

The temp sensor is a Thermistor - the cct board is inside a metal case,
I'm thinking that it might not be responsive enough if the thermistor is
inside the case.........

I could arrange to glue it to inside of case lid, or even onto outside
of case lid ........ anybody done any thermistor based projects ... just
wondered what is normal.

Are you sure this is going to work?(depends where and how you are using
the fan) fans in a closed area don't usually cool,they add heat,(from
the motor) unless they are moving cold air from somewhere else or moving
hot air which is replaced by colder air.
fans make you feel cooler by evaporating sweat, they don't actually
change air temp much.