...and the verdict is that it works perfectly... so long as the folder(s) being watched are on the host PC, not over a network like mine are
Thankfully though, there are two easy workarounds
One is to use the "run application after compression" option to invoke the DOS "move" command and the other is to move them via third party utility. I have now done the latter having used Dropresize's option to add a suffix after compression since I then have an easy way of knowing when a file has been compressed and so should be moved.
So far, so good.
Thanks for the steer, Bushfiva.
XV