it marks the high flood level
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No, it's an OS benchmark, as already explained.
D
Deleted member 137318
I did some research as I was the originator of the thread. They are definitely not high flood levels - the house I'm buying is at the top of the north downs so it would have to be a flood of biblical proportions to be that 
As <gerrydelasel> said, Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion. Sadly, I will be building over mine but I will endeavour to carefully remove the bricks and relocate them to the new outside wall
Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN.
If anybody cares, the full list (half a million) still exist here~: https://www.ordnancesurvey.co.uk/docs/gps/CompleteBenchMarkArchive-NoFBMs.zip
As <gerrydelasel> said, Ordnance Survey Bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known, the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion. Sadly, I will be building over mine but I will endeavour to carefully remove the bricks and relocate them to the new outside wall
Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN.
If anybody cares, the full list (half a million) still exist here~: https://www.ordnancesurvey.co.uk/docs/gps/CompleteBenchMarkArchive-NoFBMs.zip
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