And i'm just about to update this as well.
Is it the whole arch you are replacing or just the outer??
If its the outer section with it installed in the car is a bit awkward. Now personally i would consider removing the whole thing, then you can split it in two, replace the section that's rusty then weld it back in place.
Now the awkward bits to look for are as follows.
Fwd arch close out. Now this is only attached with spot welds but you will need to see them from within the rear arch so clean off any underseal etc.
Then you have the companion bin rear tang, again only spot welded but can be accessed from either in the car or from the wheel well.
Next will be the bulkhead panel that attaches to the inner arch, again spot welded.
Now there is the bump stop bracket in the boot, again this needs to be accessed from with the arch.
And finally the rear subframe reinforcing bracket in the boot.
Everything else can be accessed from within the wheel well.
This is the thread
http://www.theminifo...s-mini-special/