@pjs_gsy as a follow-up. After contacting support directly, and being advised to downgrade to 2.1.12, I have done that, and the problems have been mittigated.

For everyone else. I checked the channel allocation prior to downgrade, and all units are on the same 2.4ghz channel. After the up/downgrade, they are on 3 channels and everything is far, far, better.  As far as I understand, this is the problem/fix, however, reading other posts here, the nature of a fully working mesh needs them all on the same channel. My understanding of this is limited. I am a simple network engineer of 30 years :)

So, it seems like we have essentially a fix that takes the units back to more like traditional access points. The whole mesh thing seems to be immature. Not just TPLinks versions. From what I have seen of their DECO operating system and others, most are at least in part based on the same open source linux code. In seeking a working alternative to DECO, and reading the forums of other manufacturers, at least some seem to have similar issues with 2.4ghz devices. Perhaps this is just a fundamental flaw. I don't know. I just wish TPLink etc., would come clean and manage expectations. I now am stuck with 5 decos that cannot be upgraded, are missing security fixes, and I am guessing by the glacial release of fixes, this may never be addressed despite the units being fairly new.  There are many people with the same underlying issues. You just need to read the posts on this forum. 2.4ghz. Devices dropping. Network instability. Same story again and again. TPLinks answer seems to be to release a new hardware with the same OS core, or perhaps a little marketing magic feature. Some of the marketing is certainly questionable at best.

TPLink - All we ask is for some effort to fix flaws and some proper communication around that. 

  @pjs_gsy so, another comment. I did a bit more of a search about the overlapping channel issue. And, here we go. Common knowledge it seems. If so, why the hell do DLink not know about it! This is in fact the fix that was 'hacked' into 2.1.12. So, why release a 'new' firmware 2.1.14 that regressed the fix and what would appear to be common best practisce?

And, there are already many, many requests from users on this forum for manul channel selection. It would seem that the ability to do this across multiple units is absolutely necessary for reliable operation, yet as far as I know, NO deco devices have this option!

  @David-TP Thank you, David, for that update. That does indeed look promising. I do hope it makes it downstream to the previously released DECO's. The 2.14 beta unstalled and kept my 3 seperate 2.4ghz channels, so that is great. Is there anywhere I can keep informed about these new releases? A thread here perhaps? Thanks again.