Due to doing some DIY when I was tired, I managed to drop the vent for a
radiator between the floorboards. I've looked for it and can't find it
there, so have purchased a new vent. I thought it should be a
straightforward 1/2" vent/plug, but the thread seems to be different on the
radiator to the thread plug.

Am I going mad, or are there different threads for radiator plug? If so,
where do they sell those and what should I be asking for?

Thanks