Chris .. That is NOT a direct fit fan ... It is for use in ducting .. Which is probably not a solution for this application.
To replace a standard .. noisy ... wall fan, use a good brand like Ductex ... for about £25 or relocate the outlet into ceiling void so that a ducted fan (like Ductex also manufacture) can be fitted.
Kit to fit a ducted fan can be £80-100.
The fan Chris linked to is suitable for direct fit, however it has (for some reason) a big picture at the top of the page, which is of a duct fit fan, that has nothing to do with what is actually being sold on the page.