Would you be prepared to pay �10k more for your house with a bigger garden? At the end of the day, that's all that matters. Land is notoriously difficult to value because its worth as much as someone is prepared to pay for it. All a surveyor can do is draw on his experience of similar situations to come up with a figure. Paddock land sells on the open market at about �10k per acre. So on that basis this land could be said to be worth �250. It'll cost you a lot more than that in solicitor's fees. Now return to sentence one above.
Haven't got a clue why you've employed your own surveyor - CoE merely needed to have marked out what they are offering to sell using poles, then over to the solicitors.