If you recall the centre of the A and the indents of the E had turned brown.
Bailey sent me some more letters via YC Leisure and mentioned something about faulty primer,well I don't know what that was about as I couldn't see any primer.
Anyway first off removed the letters and immediately the problem became clear.
The A and the E cover screw holes in the front panel and to cover these the letters are stuck to a white backing.
This backing is obviously not UV tolerant as it had gone brown and brittle in the gaps hence the discolouration.
Now presumably what should have come with the letters but didn't was some replacement white backing ( like the picture only all white ) to cover the screw holes when the new letters were applied.
However my trusty neighbour Mr.Roscoe produced from his magical garage some adhesive plastic which did the job admirably.
I did take a photo of the finished job but it was very blurry so you will have to take my word for it that it was a success.
From the number of "brown" letters I have seen on Mk.3 Unicorns this must be a common problem.
So remember if you have the letters replaced you want some white backing as well as the new letters.