Thanks for that Roger - the Mighton winged parting bead seems to
answer my point, though I would prefer a brush style rather than
wings.


The wings may be more effective at forcing the upper sash outwards
against the box, although pushing it the other way against the parting
bead would be better. IME the gap is usually excessive due to wear on
the edge of the sash.
I tried sinking ball catches into the front of the upper sashes, and it
would have worked well had the springs been a little less rigid