@tgo I found a similar thing. M9s work beautifully with SuperHub in router mode and just plugging into one of the ports. Putting the hub into modem mode screwed things up, so have just turned off the virgin wifi. 

I guess the question I am left with is: is there any great disadvantage to this set up?...

@FunkyOtter 

I've been running just fine for the last month or so  with just the Deco Wi-Fi active, I just felt it stopped any confusion/congestion and to be honest the Virgin Wi-Fi signal is so crap that it was the reason I got the Deco to start with... I don't suppose it would hurt to have both running, I just liked the neatness of having the one (Deco Wi-Fi) running.

Although because of Chromecast/steaming issues, I did get a small 4 port switch and made sure I plugged everythnig wired into that (with the switch connected to the Deco ethernet port) so in effect ALL devices were running on the Deco rather than just wireless stuff and wired on Virgin as it caused issues with having some devices on the VM network  and others on the Deco so for me. EVERYTHING goes through the Deco only.

@tgo Yes that was my feeling. Everything I have read says trying to hook up the M9 with the superhub as a modem is a nightmare. A friend of mine works for AWS and straight out said don't bother with that. The only challenge i now have is IP addresses: the Deco issues them in a different range to the superhub. I experimented with turning off the DHCP on the superhub (my mate thought that might work) but the Deco complained profusely. So resorting to inserting a switch to ensure everything fed by the Deco, whether ethernet or wifi.

@Mcdryburn I have exactly the same setup.  Many people across the UK must have this setup.  Somebody from TP-Link could give us the best solution.  Virgin say contact TP-Link, so much for customer service.