Cheers for the replies guys.
Brand new ISP, new router, new everything from an internet perspective.
Did some research and didn't even realise my model of PS4 won't detect 5 GHz, don't know what Sony were thinking with that but that's another matter.
I disabled the 5 GHz band via the router settings and then it suddenly worked, albeit with the inevitable drop in speed as well. The router runs both 2.4 and 5 at the same time by default and I thought devices just automatically selected the most appropriate band for them? So why the PS4 can't do that with this one I have no idea.
Not at home right now but when I get home, I'll try to re-enable the 5 GHz band again alongisde the active 2.4 one and see if it throws the connection with the PS4 following it now connecting.
Cheers