I need to run a 12v fan (440w) from a 240v supply - what would be the best way?
At that power level, probably your best bet is to buy a suitable enclosed power supply such as the one pictured (600W) (despite the name 'enclosed' it will require an enclosure to cover the mains terminals etc.)



The exact minimum rating should be nailed down properly- the starting current will be higher than the run current, and most switching power supplies can behave undesirably when connected to such a load. It should not current limit when subjected to the motor stall current or you may have problems. 

The rectifier/transformer that @Charles suggests would not have the startup problem (use a circuit breaker for protection), but it would be heavier. The regulated supply might be a bit easier on the commutator, assuming it's a brushed motor rather than a BLDC motor.