Whilst perusing 'Notes and Queries' I found several references to a monolith in Hyde Park. It was apparently erected there in 1862, having been brought from Moorswater, near Liskeard in Cornwall. It was originally set up as a water fountain, but in 1887 the water was cut off.
More to the point, someone in 1901 swore they could see cup-marks near the top of the stone. In 1931 it was described as 'just east of the serpentine, in the enclosure for rabbits and birds'.
Is there a tiny tiny chance it's still there and can be gawped at? After all you don't go putting monoliths out with the recycling. Or is patently unprehistoric? Someone now or once residing in London must know.
It's got a suitable bit of folkloric-themed information with it though - a man called William Sandy was killed in an accident during its extraction. The sort of Bad Thing that might happen were you to move a Genuine standing stone.