OK, remove the new plugs and see if they're wet. If they are wet, you're getting fuel but no spark. If they're dry that would suggest that fuel isn't getting into the cylinders.

Not something I'd normally condone or even suggest but have you tried a whiff (I mean a whiff, not a saturation) of Easy Start or similar? If it briefly fires then dies, that will confirm a lack of fuel.

Take it all from the beginning. Replace with the new plugs you have, turn the mixture screws at 2 1/2 turn from fully seated, and try to start it with the tank lid open.
But make sure the petcock works fine.

Fixed! :D

So the issue was tracked down to the vacuum tube while it was connected to the petcock!

Very confusing as there was a spark, there was fuel in the carbs and the airflow was not restricted.
I had tested the vacuum tube while the tank and carbs were off and it had good suction. However, when it was all built the pipe is slightly too short and it puts a kink in the pipe restricting the airflow. So it allowed fuel into the carbs but very slowly. Of course this wasnt noticed while I tested it as It didn't have the kink in it. I have had the bike 5 years and its not been a issue before!

I disconnected the vacuum tube and manually opened the petcock by putting a pipe and sucking on the vacuum to fill the carbs. Instantly started. After a bit of a panic of working out why tick over was at 5k rpm I readjusted the throttle cable fit and its all good

Just need to book a MOT and good to go. Cheers for the help all.

Glad to hear it.

Glad it was a simple fix in the end :)