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This appears to be a mistake by the surveyor rather than any dishonest attempt to make more money out of you. The options available are to either a) agree to pay the extra £300 or b) cancel the enter agreement. If you cancel and are within the cooling off period then you shouldn't have to pay anything.
You could ask them to proceed on the basis of the original quote but if they cannot complete the job properly they could pull out of the agreement altogether.