Thanks for all the replies. I'm wondering whether the four corner holes
will be difficult to make in toughened glass.

I had a quick search on the net and found this company

http://www.diyplas.co.uk/

which sells sheets of 3mm and 4mm clear plastics that can be cut to
size and more importantly drilled (for the holes).

Can anyone think of any reason not to use this approach? seems to me to
be much simpler than getting glass cut to fit and (once again....)
drilling holes in it.