Many thanks to all those who responded - it is with regret that I am going
to have to dispose of the said tree as I in no way want to threaten mine or
my neighbour's houses.

Next question would how to do this? We could in theory chop it down - the
trunk is still quite soft and about three inches or so wide. Would this be
enough or do we have to actually 'kill' it to stop the roots from carrying
on spreading?

Once again, many thanks to you all - the seeds must have been dropped by
birds (we have an Ash in another neighbour's garden a good 70ft away). I
must say it is with regret that we are having to do this because it was just
starting to screen our garden from view of neighbours quite nicely. Oh
well.....

Kindest regards.
Kathy