"The Dell Megalith can be found in the garden area below the Serpentine. The granite boulder is all that remains of the 1861 drinking fountain that was constructed around several large Cornish stones. It is not clear whether the stones were quarried or raided from a genuine prehistoric monument. The fountain was abolished in 1897 and by 1900 only the one stone seen now remained."

From page 18 of this large pdf document:

www.hpea.org.uk/hpt/hpt8.pdf