Swapping 12v axial fan for centrifugal

Customer had a bathroom renovated and fitters put in a timer axial fan, about a metre or so of ducting but this in a roof space with no access. Whatever its not extracting well and there is bad mould as a result. The ceiling is very high, in excess of 2.5metres and the transformer is mounted on the ceiling inside the bathroom. [No, no RCD on circuit]. Leaving the issue of no rcd to one side is there a 12v centrifugal fan that will run off the transformers timer?
I spoke to Greenwood and they said all their 12v and need l and switch line and if you link them they will run until the light switch is turned off. i.e. they won't run as a standard fan off the transformer timer.
Any ideas?

 

Thank you, but not in a million years!