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I think I've finally found a reliable source for heavy grey cardstock. Someone on one of the forums suggested trying picture framers. This morning, looking for such in the Yellow Pages I found a wholesaler of picture framing supplies and a quick phone-call ascertained he was able to sell varying grades of board by the sheet. A quick visit found me walking away with 3 sheets of A0 size board in 1, 1.5 and 2mm thicknesses for a total cost of £6.71 including VAT. I also got his catalogue which includes a huge range of boards, adhesives and tapes which I will experiment with in the future.
Unfortunately, they are not on line but the address is:-
Hinton Wholesale Supplies,
68 Church End,
Cherry Hinton,
Cambridge CB1 3LB
U. K.