dg wrote:

You could leave it.

Or you could just send a letter stating that you acknowledge that there
building is crossing your boundary, and whilst it is not causing a problem
now, you reserve the right to require its removal at some future date if it
becomes a problem to your enjoyment or plans for your property. Mention that
you are not re-defining the boundary by permitting the overhang.

In legal terms you are effectively granting a licence for the use of your
land, and this licence can be withdrawn at any time.

You would also have a good claim against the surveyor - as long as it was a
proper full building survey and not just a valuation.

dg

Thanks dg, and others, & apologies for this slow response.
Obviously one thinks VERY carefully before escalating this sort of issue
with neighbours. It actually encroaches by about 20cm over a length of
about 3m. This might not seem much, but since my own extension wall is
about 1m from the boundary it blocks a significant proportion of the
available light.
I'll think about it - there's no desperate hurry ATM.

SR