How can I wire this replacement bathroom extractor fan?
If indeed the old assembly was a low voltage fan there is likely to be a transformer located nearby that stepped the AC line voltage down to the low voltage required by the fan.

On the other hand the fan that you mentioned as the replacement appears to be one powered from a 220-240V AC 50Hz power line source. The data sheet I found was here.

It would not work to connect the new line powered fan to low voltage wiring. So if your old fan was indeed a low voltage unit you have some significant re-wiring to do. 

The three wires that you describe for the new fan need to be wired to a switched power line for the manual turn on/off. The neutral connection is clearly the return path for the current from the fan. The third wire which is the "always powered" should be connected to the same AC source that is in front of the switch for the switched circuit. This always on power connection implies that your new fan has some additional capability such as a humidity sensor circuit that can run the fan when the bathroom humidity is high even when the normal switched circuit is turned off. (Note that locating the humidity sensor needs to be done with care. If there is too much air flow from the fan being drawn across the sensor it may not register the humidity level correctly).

As with any product installation you really need to carefully follow any instructions on the data sheet included with the new fan from the manufacturer. Use the information given here as a general guidance but make sure you understand specifically what you have in hand and the existing situation. If you are still unsure of the details then it may be best to call in a professional electrician.