Pictures 1 2:- Did some gentle sanding of the decal, going as far as that shown in picture 2 - with the most damaged part removed. I left the rest of the decal on as it was sat well onto the fuselage. The dark green acrylic top coat of paint was starting to come off with it as I sanded and I didn't want a big aluminium area left accross the fuselage. (Spot the crease on the white band).
Picture 3:- The new decal in place, overlayed over the top of the remaing old one. It matched up very well. The underlying paint does show through a bit but overall - very pleased with the result.
Thanks for the advice Wayne, it was definitely the way to go.
Pictures 4 5:- The yellow leading edge markings added together with the cannon muzzles and pitot tube.