Nice to talk with you Gaz: for general info;
Unfortunately wear to the anodising on RC39/40/41 stanchions WILL occur[and also the gold and black Ti-Ni of the older forks], notably starting on the disc leg front face-slightly toward the wheel. Problem is that the torque twist of the brake opens the seals allowing muck in. This is held against the stanchion and wears the anodising, pressure from the bushings just below the seals gives the stripey look and splined feel. If left unserviced the muck then contaminates the bushing which then chew away at the stanchion faces; in extreme cases down and into the alloy itself. If the forks arent regularly cleaned/serviced the wear will appear more generally to both stanchions eventually, but is ultimately unavoidable.
To extend the life of the stanchions the seals should be regulaly inspected to make sure they arent holding a load of sand, soil and silica between the upper and lower lips-ping the garter spring off and peel the upper lip back and clean any crap out with a cloth. Better still remove the carbons and clean the seals and refresh the lube in the legs[beware of oil spillage though!].
If the stanchion surface anodising is still intact but has just lost colour then the performance will not be affected, it is a good idea to replace the upper bushing which will be contaminated; this will again extend the forks life.
If the anodising has worn through to the alloy then the fork will still operate fine but the bushings will eventually dig a great hole in the stanchion face and the fork will leak oil and take in more dirt and the fork will have a limited life.
BTW forks from different areas of the country show different wear characteristics-something to do with the viciousness of the differing soils/sands.
Hope that is of some use/interest