Replacing manual window regulator

[PeterF]

Hi guys, I've just got an old 2004 berlingo with several problems that I'm doing up as a camper, so I'll probably be asking a lot of questions here in the next few months! 
The first job I'm attempting is replacing the broken drivers window mechanism. It's a manual window and stuck in the up position. For some reason the handle refuses to turn. I've had the door apart and can't see anything obviously wrong, so was planning on fitting a new unit but it looks like it'll be really awkward to remove and refit. Do you need any special tools or anything? Is it as hard as it looks?

Thanks in advance!