Ahhh yes, that's the command we use (when required) on all Ubuntu Packages, but had wrongly
assumed, that Plesk was package managed, independently outside of that. Our own, basic school boy error then... this explains why we couldn't find any references within Plesk as to what we'd assumed and this also negates post #13 in this thread.
Okay, well that was easy to correct and all those warnings have obviously now ceased. Thanks
@Kaspar@Plesk