Thanks Johnson, bobbyv & dlswnfrd (Don?),

I've had a close look at the extent of dullness, and it is fairly evenly dispersed...top and bottom.
I appreciate your point, about the delicacy of the front silvered surface, Don, and I'm concerned too about the holding clips being brittle. Looks like new units when I can afford them! Thanks to you all.
Murray. (NZ).