Adam I did this mod last winter,
I found the only way to get the old one off is to cut it, I tried various cutting methods, even down to the wife's secateurs, in my frustration. Perseverance is what's needed, oh and plenty of coffee too.
Putting the new one on is easier but not that easy to put the bolts in if I remember correctly some have captive nuts and others you need to put a nut on. If I remember right I ended up with one nut taped to a large flat screwdriver a real pig of a job but it sorts the water leaking problem out.
Oh and mine's a Mk II as long as it has the box shaped inlet it'll leak, particularly if you park it facing uphill.
Prior to refitting I would suggest ample mastic around the welded joints as nibe leaked there too.
Well documented on .org I think
If you need more help give me a shout I will pm you my moby number
Peter
Plus a little extra is that although Meg's air-con is STILL only half installed I found the connector that powers up the front unit - and voila - recirculated air!