H.V.Skan Ltd 425/433 Stratford Rd Shirley, Solihull West Midlands B90 4AE 0121 744 6791
I do not know if they are still in business, and they were rumoured to be expensive. You could probably also get it straight from
Schott Optics Division Sales Dept. Optical Glass P.O.Box 24 80 D-6500 Mainz Germany
You might also try ringing telescope-making firms advertising in Astronomy Now; one (I'm sorry, I cannot remember which, again it is some years since I enquired) had slabs of Suprax (like Pyrex) up to about 14" that were reasonably cheap. They will also usually supply grinding powders etc.
Roger Moss, department of Engineering Science, University of Oxford.