Ooooh that will be worth more than you think
I was involved in the new Coventry hospital build and the word was that the client paid £5 million to buy about 20 acres of land.
Obviously not the same league, but your land must be important to them and the health and safety aspect suggests it's somewhat a necessity.
It's a suck it and see thing, IMO.
The nature of the beast is that they want it for nothing, so you should screw them for anything you can get out of them, even though you don't have the mentality.
My experience is that you should tell them to tell you what they are offering and you offer nothing. If you're happy with their offer, put 50% on top and offer that.
Reinstating the wall is totally down to what you want and could be an improvement, as is your shed and anything else
As for actual amounts, it's simply down to what you can get.
Whatever, get it all in writing.