So ... spent all afternoon accessing the ATU, checking, tightening, cleaning (several now bright and shiny from sanding!) all radio connections, including checking inside the ATU -which looks fine (that took a couple of hours to take apart and then get the cover back with the gasket properly in place).
The result? NO transmission possible on ANY frequency now!!! Even tuning sounds my SWR meter alarm. The only place I've not yet checked is the most likely culprit - the feed-wire-to-backstay connection. That's high up above the aft deck and I need the boat to be still to perch on something to reach there - likely to have to go back to the marina to sort that out.
No radio use until this problem is fixed!
Actually thought I was wrong - tried radio and all seemed fine on every band except 20m (tried that last) -but when I re-tried after that, all other frequencies sounded the alarm - so we're back to square one...