The ground surface behind the 5 foot high stone wall in the back of
garden is about 5 foot higher than my garden surface. The wall was built
on a 3 or 4 inch foundation of broken stones. There was no cement put
into the foundation.

Tree roots from the ground behind the wall (there's a line of quick
growing conifers about 4 foot behind the wall, and there's a Finish
White Beam and a tall Poplar tree) have squeezed under the foundation
and then proceeded to fan out over the surface of my lawn and damaged
it.

Below about 4 or 5 inch of the ground surface I have sandy stony ground.

I'm redoing the lawn and the wall. I'm going to keep the old wall in
place, and put a brick wall in front of it with a proper cemented
foundation. This new wall will be spaced about 2 inch away from the old
wall.

I've got some pavement flags (slabs) 2 goot by 2.5 foot, 2 inch thick.
I'm going to put them in the ground adjacent to the base of the old
wall, with about 6 inch sticking up above ground surface. So, the flags
will penetrate 1.5 foot below my ground surface just in front of the old
wall. Then I'll fill the 2 inch gap between the walls with stones.

Okay, is this going to divert any roots downwards from under the old
wall, into the sandy rocky soil and stop roots growing on the surface of
my lawn? TIA.