Another London erratic

Found this on Tim Daw's interesting web site:

This stone was found in 1899 in a gravel pit on the site of an ancient river bed and moved to its current site just inside the gate to Elthorne Park in Hanwell, west London. It is roughly 18" deep and 5' square but was formerly much larger as large chunks have been knocked off 3 of its sides. It was glacially deposited and belongs to the middle division of the London Lower Tertiary Sandstones. It is of similar age, origin and composition to the Sarsens of Stonehenge.

There was also this comment from Dave Coleman:

I spent most of my life in Harrow and this Sarsen was the middle of three sites that I knew.
Just over a mile north, there are still two stones, today, outside The Hare public house - crossroads on the edge of Harrow Weald Common.
Just over 1+1/2 miles south was the other - outside the Havelock public house - central Harrow. Both stone and pub are now lost.
(I was always told that these stones marked the funeral route to Stongehenge for the burials of ancient Saxon Kings).

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This is generally referred to as a "glacial erratic" but I'm not so sure -- it is similar to sarsen, and there are a lot of residual sarsen stones littering the landscape of SE England -- most of them having been emplaced by non-glacial processes.......

The boulder is currently located north of the Thames and north of the M4 motorway in west London, so this is well to the south of the glacial  limit in this area.  Not sure where the original "gravel pit" find might have been.....