The plan is to increase clearance for bigger wheels? There was a discussion a while ago about doing this a different way which has some ideas (and a link to Beetle photographs) http://www.mig-welding.co.uk/forum/showthread.php?t=169
To just enlarge the arch I'd be inclined to panel beat. Just draw a line on the wing where you want the edge to be, then another line 1/2 inch further outboard for the flange. Cut along the second line and then use a hammer and dolly to knock the flange around a little at a time. Probably the shape of the wing will change a little in the process so might need a little knocking out with a soft hammer and tidying.
Hi Malcome and thanks for the reply on this. I actually need to cut and reshape the mudguard/ fender itself by taking a few inches off the way it hangs down over the tyre the way it does. I think its a home made one, but just needs a bit of trimming to make it look tidy, was wondering if there was a particular way of doing this without making a mess of it!
Puzzles me as it definately doesnt look right, just a daft suggestion here 2nd hand wheel wings are pretty cheap to pick up, might be easier going down that route and then just experimenting with the wing? I know wheres the fun in that Sorry I'm really not experienced enough to give further advice