According to Wikipedia:
- In 1750 (approx.) the approx. 0.6m-tall half cube on a tall stone pillar was moved about 500 metres upstream to a site at grid reference TQ028718 by the river in the Lammas Pleasure Ground.
- In 1986 the stone was moved to the Old Town Hall Arts Centre, Market Square and a replica was placed in the Pleasure Ground.
- In 2004 the original was moved to Spelthorne Museum, Spelthorne Library, Friends Walk/Thames Street.
The current locations are definitely right, as I found the old one (above) in an annex to the library and the replacement one out on the Pleasure Ground by the river:
The stone used to be out in Lammas field which is worth the short walk out there as its very pleasant:
Staines was chosen as it was the highest point of the river that a tidal variation could be detected: nowadays there is none due to tidal locks downstream e.g. at Teddington.
From Staines railway station its a short 1.5km walk there and back according to Google maps:
The stones are well marked with useful information boards, for example stating that:
The London Stone once marked the upstream limit of jurisdiction over the River Thames by the Corporation of the City of London. The Corporation acquired these rights from the Crown in 1197 in the reign of King Richard I and held them until 1857, when the Thames Conservatory was formed.
In almost seven centuries the Corporation enjoyed many privileges, such as charging tolls and taxing fish traps and was also responsible for this vital artery of trade being navigable.
The one in museum explains why the date on the stones is 1285:
It was ordained in 1285 that the Lord Mayor of London should have the management of the Thames from Staines Bridge to its Estuary. The limits of jurisdiction were marked with boundary stones, of which the London Stone is one. These rights are confirmed by the date 1285 which appears on the mark stones at Staines and Leigh in Essex.
Hmmm.... is it Leigh or is it Southend-on-sea? Time to head east to find out....