In reply to Atholl de-Saint-Croix:
Sewing the patches on may have one of two consequences, depending on how you do it:
i) sewing through the patch, then taking the thread outside the patch edge, through the shell fabric, and then back up through the shel fabric and patch (step & repeat) wil hold the edge of the patch in place, and hopefully stop it snagging on things, but is likely to pull holes in the shell fabric.
ii) sewing only through the patch & shell fabric, and never taking the thread outside the boundary of the patch (using a simple up/down running stitch) is less likely to pull holes in the shell (because the patch is likely to be stiff enough to resist any movement of the stitching), but the edges of the patch may snag on things.
The best compromise may be to combine i) and ii), so that, rather than bringing the thread outside the patch and through the shell, sew through the patch and shell as close to the edge of the patch as you can. You should end up with a zig-zag stich, with only the 'zig' visible on the surface of the patch, and 'zag' stitch inside the shell.
If the patches are iron-on, you might use the methods described in my article on repairing waterproofs, either using the hot-melt glue alone, or in conjunction with the sewing. Iron-on is likely to be pretty much permanent; sewing at least allows the patch to be removed.
http://www.outdoorsmagic.com/news/article/mps/uan/6509