Seems nearly there, Tom…
If I do it like that and leave the unintuitive, need-to-click-the-top-level-link-to-see-the-page behaviour, it works perfectly.
But, if I create a new, non-clickable custom link for the top level and move the original ‘shop’ page to the top of the drop-down (which is what I’d prefer to do as more intuitive for users), I lose the current page menu styling from the new custom link when I’m on the main shop (products) page, and haven’t worked out to get that back yet.