Lobster wrote:

What I'd like to do is fix the upstand in position with the gap behind
reduced to 5mm - with the right filler, that will look OK, doesn't cause
the other end to bow outwards, and doesn't require *too* much bending
force.
Any thoughts on any of this would be most welcome.

Would it not be best to scribe then cut/sand the edge of the worktop to
the wall rather than try to fill a gap?
Place the work top up against the wall and then take a piece of
timber about 1 inch wide and 1 or 2 inches long -depending on how uneven
the wall is- and fix a pencil to the end by drilling a small hole and
inserting the pencil into it, making sure that it is a tight fit.

Using the wall as the template you then place the end of the piece of
timber without the pencil against wall and slowly drag it along the wall
making sure that the pencil point on the other end is in contact with
the surface of the work top, as you scribe along the pencil will mark
the outline of the uneven surface on the worktop. Then cut/sand to this
line and the worktop should fit perfectly to the wall.
Or have I totally misunderstood the problem?