There are no way of establishing a 'value' - it is as much as you think it is worth.
What to do: Have a discussion with the neighbour along the lines of "your desire to increase the amount of veg you grow in the garden - the healthy aspects and what with the credit crunch. Would he sell you a piece of land?"
What not to do: "Its a real pain having to park my valuable black taxi on the road and if I could access around the back, I could park in there."
The difference? - probably �2000.
The secondary issue is resolve is employing a solicitor or licensed conveyancer for you (and for your neighbour) to effect the sale of the land - you would need to get it registered at LR otherwise problems could occur later if you just move the fence.