iOS - Certain games have patchy connections, resorting to 4G cellular network resolves connection issue.
iOS - Unable to connect to VPN service (Express VPN), again using a mobile hotspot from my 4G phone resolves the issue.
Attempts to resolve
Power cycled all devices - multiple times.
Investigated uPnP settings, tried the less secure setting but this didn't address the problem.
Contacted Plusnet support via chat, they reported no faults in my area. They couldn't tell me if my router had a firmware upgrade over night however.
Without any means to fix and not wanting to do port forwarding based on the fact that everything was fine on Saturday and this is impacting multiple devices I signed up to OpenDNS and changed the DNS settings on multiple devices.
Everything now works!
No issues with PS4 or iOS devices, in fact responsiveness looks to have improved all round.
Plusnet, if you're reading this please take a look at this DNS issue. Your hub doesn't allow me to edit the DNS settings so I'm now faced with having to buy a different modem / router to address this issue.
I agree that Plusnet should allow setting specific dns servers on the hubs. This kind of problem crops up every now and then intermittently, which is supposedly being looked in to. Your options are buy a different router or the cheap option (but time consuming) setting DNS servers on all your devices.