tan - the small amount showing in your pic is of little value; new 70mm x 3600mm costs about £5 each & 50mm x 2700mm about £3 each. Some of yours looked buckled - fit for the skip.
Do you really think they would have left it behind if it was of any value? Unless it was in the contract to leave such material I would be on to my solicitor to forcibly get them to dispose of it for me at their expense.
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