Yes, I thought as much. I had it set as 10.20.30.0 to begin with but deleted the itnerface and started again and copied yours. Your explanation of how IGMP works makes a lot of sense.
I'm seeing packets on the IPTV_WAN interface which I wasn't before, just still getting the IPC error on the BT box with IP channels like Sky One.
I don't have any switches between the Opnsense box and the BT box - it's straight from the back of the router into a patch panel which the BT box is plugged straight into. I couldn't get my switch to behave properly with the VLANs so thought I'd just separate the traffic altogether.
I might try the old classic of a reboot to see if that helps anything along!!
Thank you so much for your help, I'm not quite there but have made great strides in the last few minutes
Just managed to get this working, thought I'd come back and leave the settings that worked for me in case anyone else finds them useful.
The setup etc is as above, the firewall rules and everything else work fine. I kept seeing an error in the logs for the IGMP proxy showing an IP address that looked like it was in the range but wasn't being allowed through.
The IGMP upstream addresses that worked were 184.108.40.206/4, 220.127.116.11/24. I had the second one as /32 which gave this error "The source address 18.104.22.168 for group 22.214.171.124, is not in any valid net for upstream VIF."
Hopefully you'll have a bit of luck sorting it out too!