I already have a working extractor fan in my bathroom that i would like to replace. Am i right in thinking that i can just buy the item in the below link and then have my electrician connect it up to the old flexible ducking hose?
Only NEW circuitry has to be installed (or, at least, inspected) by a qualified electrician. Replacing electrical items (such as plug sockets, light fittings and extractor fans), using existing wiring, is something which anyone can legally do.
As others have pointed out though, your link doesn't go to a fan anyway!