So...how do I dig/pour the foundations for the wall for my extension
so that the wall of my extension adjacent to theirs has zero clearance
and yet sits in the middle of the foundations when it is built and
also does not involve going onto their land/property?

I think when I start digging at some point I 'might' hit their
foundations for their extension on my side of the property.

This is exactly the situation I had when I began building my
extension, the only exception being that I have a very good
relationship with my neighbour. If his extension has been built
correctly then it is likely that his foundations are actually on your
property, but they are unlikely to be wide enough to take your new
walls. What I did was to dig out to the required depth along the
length of his foundations and when pouring the concrete we had to
overfill to a specified depth on top of his foundations. This was
only required along the neighbours wall so there was in fact a step in
my own foundations along this length. I cannot remember the exact
depth, 100mm springs to mind but I could be wrong. I had dug out
already and when the BCO inspected the diggings, it was then that I
was advised how to proceed.

I would be more worried at how you are going to proceed when you get
to full height if he is unwilling to grant you access, you may find it
difficult to make good at roof level if you have to do everything from
your own side.