Ferroli Modena 80E Boiler Noisy Fan?

Hi there,
I've got a really noisy boiler fan. It's been driving me mad for ages so I got soemone in to check it. I've been advised that it's not causing any problems but if I want it replaced they can do it for £100's. I can buy one on Ebay reconditioned for £50ish and would rather fit it myself. I've taken the covers off the boiler and it seems fairly easy to connect the fan together but I don't know how to actually remove it from within the boiler. Can anyone advise? I'm fairly technical (I do all the work on my own cars). The fan seems to be connected at the top under the flu and it looks like a screw at the front but I can't get access to it with any tools I have. Is there some special tools required?

Any tips and advice on how to remove this fan would be appreciated. The thing's driving me crazy.

Thanks