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?

Contact the local council and ask how much they would charge. We've got a
three foot strip running across the back of our garden to prevent
developers knocking a very nice plot with 3 houses and a small block of
flats with lock-ups into a housing estate.

If it were me, I'd ask for the value of the proposed buyers house, and get
a covenant put on it such that it must be fenced/maintained, and only used
for foot traffic by the owners of the now buyers property.

But then I'm like that

Al.