Thanks for the reply...in a couple of places, the mirror finish has rubbed off...it's a cosmetic issue, really. I have taken the platter to a mirror silvering shop and they were unable to help. A normal mirror finish is applied to the underside of the glass, with an opaque, protective backing applied. Left to the 'elements', the mirror silvering would a) tarnish very quickly and b) rub off to the touch.
This left me concluding that the effect was a top surface mirror. However, I have sent the platter to the only (?) place in the UK which provides top surface mirrors, to be told that the process employed by Nakamichi is NOT a top surface mirror, and that the finish was - somehow - blasted onto the glass...they have struggled to remove any of it. I find it hard to believe that, despite 25 years of technological progress, the platter is defying explanation!
I would like to get it sorted, even though I am enjoying the sound of a stainless steel mat much more, and the centre search works fine with it!