With the KODI issues still unresolved (ignored?) https://forum.dvblogic.com/viewtopic.php?f=66&t=31841, I've taken another look at the Apple TV client.
I noted that AppleTV can "learn" specific remote commands (See pictures below). I have now learnt these commands (and others) and tested use of the AppleTV video playback, PLEX and other applications and all work with the Harmony Remote using the AppleTVs learnt commands.
I can now control the Apple TV with my Logitech Harmony and no longer need to use the annoying gesture pad on the AppleTV remote .
...but...
DVBlink client doesn't seem to support this standard AppleTV functionality Will you include this in a future release? - I hope so.