Thanks Frank, that puts it into the timeline for either Chance or Webb. I'm assuming then, that Pirelli bought in pre-slumped trays, decorated and fired them. I suppose it's not inconceivable that Pirelli had the equipment to slump trays, but I have no idea how large its set-up was.
Would it be reasonable that Pirelli approached Webb Crystal as it was based in London, not too far from them in Potters Bar? I think importing a cheap item would not have been very cost-effective, even in bulk. Or was it a simple re-badging exercise: made by Chance/Webb, stickered by Pirelli?
All hypothetical suggestions, of course!