hard to judge without all the facts and reviewing the documents, but if its a once off lump sum you've good arguments for cgt treatment. it is a grey area so i wouldnt be too harsh on your 2 "experts".
although its not a deciding factor you might ask the quarry company how they will treat the payment from a corporation tax point of view - ie, as a capital payment for the acquisition of the asset, or as a trading expense.
why not insist on an indemnity from the quarry company (ie, if income tax applies they will be liable to an additional payment to bring you back to the position you'd have been in had cgt applied) - that will incentivise them to draft everything as if cgt applies!