Richard wrote:
"I want to work 160, 80 and 40 with a 20 metre length of wire (fed into
an atu) that is to be supported by a mast and a pole on the house
chimney stack."

The wire to earth from your atu is a part of your antenna even if you
don`t ground its far end. If you desire a strong vertical component to
your radiation, put as many radials in your garden as possible to
minimize the distance displacement current flows to reach a copper earth
return wire.

Best regards, Richard Harrison, KB5WZI