In reply to Grim:
Hi,
I use y&y belay glasses over the top of my specs and they work fine. I've also used the CU belay glasses, which again work well. I've not noticed any scratching problems, but then I don't wear £500 glasses either!
The CU belay glasses are definitely a superior product, but then you'd hope so at twice the price of the Y&Ys.
HTH,
George.