I managed to order the wrong style of actuator - body correct, but the
plastic plunger was around 6mm shorter. Wattage was also considerably
less than the original. Rather than mess about further, I decided to
see if I could adapt it to work.

I drilled a 0.75mm hole axially down the plastic plunger and Araldited
in an hardened steel pin, I then trimmed it down by trial and error
installed in the fan. The actuator didn't get even nearly as hot as the
original, so I didn't bother fitting a diode. Rather oddly and despite
drawing less current, it opened the shutter quicker than before.