according to Christopher Chippendale in 'Stonehenge Complete'...
"Another single sarsen,blackened by smoke, stasnds in Hyde Park just east of the Serpentine, a forgotten relic of a 19th century bridge of sarsens"
I haven't been to the park since reading this, so I can't give any better idea of exactly where to look, but I think it shouldn't be too hard to spot when you know it's there.