If your was stuck with one IP (only using IPv4 here with one static ip) Then theses a alternate port selection on the Xbox itself (not 360) as by default this talks to 3074 so could be this is conflicting

Dashboard > Settings > General > Network Settings > Advanced settings > Alternate Port Selection > Manual

You have to do each Xbox and then port forward the manual port and remember to set DHCP lease.

Remember if you have any QoS make sure you 1st prioritize them by xbox UDP ip source port rather then the destination port, And then make a QoS for any destination port between UDP 3074-3079 as this will help
Also no UPnP enabled on router.

Edit:
i forgot to mention, This wont work for backward compatible games so make sure port udp:3074 plays the most backward compatible.

Edited by francisuk25 (Tue 23-Nov-21 12:15:51)

I spoke with Zen last Friday and they put the request in with CityFibre Monday morning. Had an update yesterday to say the request to enable ipv6 had been actioned and they would update me when CF had done it.
Having read up on ipv6 a bit and got the basics, I believe this will resolve my issues as NAT is no longer used and effectively each device gets its own IP within the static range supplied by Zen.

That's interesting that Zen have to ask CityFibre about it - would suggest there's an element of L3 that CityFibre are responsible for, and they don't just hand the subscribers over to a Zen-operated switch.

Finally got an email this morning to say IPv6 had been enabled. Rebooted everything and now works perfectly. No additional config was required and just left everything as default.

Thanks All