@AndyL61 and
@Leni16
FWIW the oil leak was a known thing when I picked up my car, but the dealer released the car unmodified (there was no official advice to do otherwise from MG at that point). I checked carefully before and after my drive home (about 150 miles), and there was nothing. The leak first appeared after a three hour cross-country drive (it was winding, undulating roads but my Wife was in the car and drove about half of it) at about 500 miles in total.
A reduced oil level and a good clean and its not been seen since.
So just be careful to keep looking, if it isn't there when you pick it up there's no guarantee that it'll not appear later, so keep checking.
The oil can appear from either side of the undertray and there's no easy way to see the breather in situ, so the delay is not unexpected. I don't understand the mechanism that caused the issue on a lower speed run - the twisty roads clearly did something that constant (relatively) high speed didn't.