I've just had a closer look at my old one, and the O ring gasket is still there
However, the gasket is sitting flush with the plastic on the bottom of the cap, and not making a proper seal with the rocker cover.
On the new one, the gasket is nice and thick, and sits proud of the plastic on the cap. Would be a lot cheaper just buying the gasket, as they said they sell that separate, but easier and quicker to just buy the whole lot.