Thank you all for your input. I think I have obtained a better
understanding of the issues involved, and for those of you are
interested, the following is a summary.

The simple answer lies in the Town and Country Planning Act 1990. In
fact the application form is quite clear on the subject (although the
Planning Officer didn't advise me about this). The form contains a
certificate of ownership under Section 66 of the TCPA. If the
applicant is unable to certify (Certificate A) that "the development
does not encroach onto neighbouring land or property (including
footings, gutters, etc)", then he has to certify (Certificate B) that
he has given notice to everyone who owns any part of the land to which
the application relates. Well in my case, the applicant ticked
Certificate A which was incorrect - he should have ticked Certificate
B, having previously advised his neighbours that his proposed
development would encroach on their land. It is only after he has
served notice to his neighbours that they can consent or not to the
proposed development, and as some of you have said this is a civil
matter between neighbours - the negotiations can then start.

So my Planning Officer was not entirely correct to tell me that the
encroachment is not a planning consideration - he should have picked
this up and should not be proceeding with the application until it is
sorted out. Maybe he would have picked this up on his site visit, but
that will normally occur after the deadline for all objections has
passed.

HS