Just checking back in, five months after I first started this thread.
Went through many different routes in an attempt to sort out my issue, culminating in selling my TP-link Deco posts on, thinking that their incompatibility with the EE Smart Hub might have been at the root of the issue (when they weren't connected and I was simply using the main EE Smart Hub Wifi, the router behaved itself).
I attempted to upgrade my EE subscription to the full package that included EE's own Wifi booster system but, after a month of EE not sending the Wifi boosters to me (another story for another time) I gave up and looked to try another Mesh system that might play nicely with the Smart Hub.
I ended up getting a newer set of TP-Link Deco posts (X20 AX1800). But - and I think this may be the important bit - before I set them up I found a page, hidden away on TP-Link's support pages that explaining something called Ethernet Backhaul which appeared to match the configuration that I was attempting to set up.
https://www.tp-link.com/uk/support/faq/1794/
My home network is mainly wired between the three floors of my property, but I wanted a Wifi mesh post on each floor, connected to this wired network, which would broadcast this strong wired signal over Wifi. In order to do this, I had to initially connect each of the Deco posts together by an ethernet cable connected directly to the SmartHub until they registered the Backhaul mode (or something), then disconnect them once they'd gone into this mode and then reconnect them to the wired network via network switches on each floor. When I looked at the network diagram on the Deco app on my phone, each post then showed as being directly wired to the main network and were all broadcasting a strong signal via wifi. The main takeaway from all this is that the SmartHub stopped falling over and I haven't had to restart it since.
This probably means that my original set of TP-Link posts would have probably worked fine if I'd have seen how to set them up properly in this way. Ah well, sometimes you have to go round the houses to get back to where you need to be.
To confuse issues slightly, while I was troubleshooting the original issue, EE support did set me up on some new SmartHub firmware. However, I don't think it was solely this that fixed the problem I was experiencing. I think it was setting up the backhaul mode on the TP Link Mesh system that made everything tickedy-boo.
