i'm still unable to sort this issue,

so the tempory fix is that I've removed the old metal ring completely which leaves it with the plastic filler neck exposed but obviously no thread for the original cap to screw into.
ive got a emergency filler cap for now to go in the filler neck to at least block it off.
i still need to get this sorted so if i find out any more info i'll post it up.
looking like a used fuel tank may be the only option for this fix