TrigpointingUK

I note that there have been a few unsuccessful visits recently looking
for the Surface Block at Ambwell http://trigpointinguk.com/trig/8428
(SD 90201 42740, TP8428)

By chance I found a reference to it in the benchmark records:
https://www.bench-marks.org.uk/bm99129

RIVET Foulridge, Ambwell Trig Station SD90204274 Other Unknown 390200 442740
RIVET: NBM RIVET CONCRETE BLOCK 9.1M W WALL JUNC 3.9M S WALL (AMBWELL TRIG STAT
ION)
(ODN 300.9812m, AGL 0.0m) RV5841 99129 2018-08-27 53.88077338 -2.15056241

So with luck and a good tape measure, the distances of 9.1 metres West from the
wall junction and 3.9 metres South from the wall ought to fix the location.

The sketch for the Ambwell trig station obtained from the National Archives.
This shows a measure of 8.7m W along the wall from the wall junction to a reference point.
Then a measure of 13.7m S along the other wall from the same junction to a 2nd reference point.
The length of a line between the two reference points is given as 17.3m.
Measuring from the NW end, the trig should be 2.9m along the line.

Interesting that the sketch is not consistent with the measures given in the benchmark database.
The sketch also refers to a "White Building on Hill 1 mile"
but this isn't particularly helpful as several buildings might be the one mentioned.

Site visited on 22 June and obtained measures matching the sketch.
Multiple gritstones unearthed and moved to the side of the wall.
Ground quite hard; unable to probe beyond ~3" down.
Nothing found; gave up after about an hour.
But its possible that the block still present; either buried deeper or under a stone.

Sketch added to trig wiki:

http://trigpointing.uk/wiki/File:TP8428 ... Sketch.jpg