Ordnance Survey Benchmark

You may have seen one of these before on your travels. This is the one on All Saints' church here in Milton. It's an Ordnance Survey benchmark, a type called a "cut mark" one, i.e. it's carved into the stone. If you look it up in their database it's 7.646m above sea level and 1.1m above the ground and it's a secondary mark. Last levelled in 1971. Verified in 1984.

Having read up on this then if I understand the maths correctly the nearest "fundamental benchmark", which is considered error free, is at Wicken, which is 11.35km away and hence the error on this benchmark¹ is:

Uh huh. I'll bear that in mind when I'm trying to keep my nose out of the water.

1. As it's a secondary, it would ±2mm if it was a Geodetic and ±12mm if it was a Tertiary.