Bit OT but I'm sure within this group's expertise...

My parents have recently been approached by their next-door neighbour
who wishes to buy a strip off the end of my parent's garden, presumably
with a view to gaining rear access to their property.

My parents just *might* be interested in doing this. One of several
questions raised, though, is how would you put a valuation on this bit
of land? From memory, my guess is that the land would be of no
particular use to anybody else; ie not large enough to build a separate
property on. So I really it's a case of how badly does this guy want
the land, and how much my parents would miss it versus how much they'd
like the cash realised... but where on earth do you start in terms of
coming up with a figure for something like this? My parents (and I)
literally don't know whether 100, 1000, or 100,000 GBP would be the sort
of expected value. Is it something an impartial professional can advise
on? Or is it simply a case of my parents coming up with a figure they
could live with, and the neighbour liking it or lumping it?

Thanks
David