I found it a bit fiddly, but they do keep a lot of muck off your fuel tank and chassis. Better again to fit the mud flaps extensions that stop anything getting behind the wing/on top of the chassis. The extensions are for sale at Glynn Lewis, but you can make your own for a few quid.
Fitting went pretty much like this : Place the bracket against the outrigger and drill the holes. IIRC the fiddly bit was getting a bolt down through the outrigger after drilling the holes. I used a bit of tape and a short bit of wire to put them in place - tape bolt to wire, use wire to guide bolt down through hole, grip end of bolt with pliers, pull wire/tape off. If you have small hands you may be able to reach with your fingers instead of this. Then just place the bracket over the bolts, and put the washers/nuts on. If you can't get a spanner onto the top of the bolt, this works: grip the upper end of the bolt (the bit sticking down below the bracket, which you are holding in place with your third hand) with a thin pair of pliers, get the nut and washer on until you can see threads below the nut, grip the threads below the nut and tighten more. I recommend using stainless nuts and bolts, and lots of copper grease.