After a bit of fiddling I got this all working nicely, so I thought the info could be useful to others.
EE Smart Hub Plus IP: 192.168.1.254
Raspberry Pi IP: 192.168.1.253 (static IP)
I did a fresh install or Rapbian Lite on a RaspberryPI 3B.
Installed PiHole and setup some basic blocking, initialy using 1.1.1.1 (Cloudfire)
Turned on DHCP within PiHole, server with the range 192.168.1.50 - 192.168.1.250 (This also sets the DNS to use the PiHole IP)
Turned off DHCP in the EE Smart Hub Plus.
Installed Unbound in the Raspberry PI and set PiHole to use Unbound for DNS resolution.
Added the Deco Mesh in access point mode, so it's DHCP is turned off.
Turned off EE Smart Hub Wifi (both bands)
Now everything connects and gets an IP from the PiHole, either wired or wireless.
All traffic goes through the PiHole (Currently blocking about 30% of requests)
And web pages seem significantly faster loading.
