Sorry guys, belay that request. It was a false alarm.
It would appear that I was victim to a strange optical effect. My examination was made with strong sunlight coming from the window behind the cartridge in line of sight. Viewed in this way there appears to be a dark circular "tide mark" which must be due to a degree of translucency in the red plastic.
I went back later and shone a torch on the front face and it looks new under those conditions but there was at least one large flake of vinyl material in the bottom corner of the front face.
Recently I've been playing new vinyl without cleaning and it's possible that pieces of core material i.e. rags from drilling of the spindle holes, may have ended up on the surface of the disc. When the stylus makes contact there is a chance they could ping off, statically charged, and attach themselves to the nearest object - record, stylus or front face? It's one theory but these particulates accentuated the illusion of vinyl residue. No way do these few particles form any kind of a circle when illuminated... :)