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.