Thanks Beano. I'm in Newcastle and there's a place nearby called Specialised Plastic. I'll give them a try. The face of the cluster is pretty grimy, The coloured part , that is. I'm struggling to get it clean, but I'll keep tryingI have installed new perspex covers for the dials in a few 504s, and I recommend doing it, if you can find a place that has perspex offcuts. Just look up the yellow pages. And they don't even have to be cut perfectly...a little large or rough outer edge won't show up, though you may have to tack them in place whilst installing.
I used to also rub back the plastic case with 800 grit sandpaper and repaint with a spray can. Used to look like new.