How to change noisy Fan Glow Worm ?,Could Please HI-Tech plumbing help?

The fan on my wall mounted Glow Worm ultimate 40 FF is very noisy. Just got charge £60 for an engineer to tell me what I allready knew and will charge me another £75 for changing it. Could someone please help with instructions on how to change this fan and where to purchase it at a reasonable price?

Thank you so much in advance.