Rob writes
Tree type - I really don't know - it was from Aldi for a fiver, and
I've thrown the label away. I'm pretty sure it just said 'cherry tree'
with a picture of a tree with that pink blossom. It had a black tiny
beetle infestation early on, accompanied by ants. I saw that off with
systemic insecticide and greased paper around the trunk.

Placement - it gets worse. It's about a 2 feet from a substantial 10
foot wall/bank down the to the house below. My plan was to just keep it
at about 7/8 feet, with a misplaced notion that the roots wouldn't do
anything spectacular. Thing is, there are lots of trees along the
boundary line of other houses. Trees without substantial route systems
I suspect.

Looks like an uprooting is in order? Some pics:

Just because something will eventually get too big doesn't mean to say
you have to uproot it straight away. You can always enjoy it for a few
years then get rid of it.

But I would be a bit worried about that retaining wall, and I'd want to
make sure the roots couldn't start damaging that - hopefully others will
be able to advise you on creating a barrier, or replanting the whole
thing in a buried container.

--
Kay