This appears to be the datum line of an OS cutmark. The OS record the existence of one of their benchmarks on this cottage, but it wasn't at the described location, and we concluded that the original brickwork had been taken down and used to rebuild the wall. The associated arrow was nowhere to be seen.
Uploaded to Geograph by Richard Law on 20 October 2016
Photo © Richard Law, 20 October 2016. Licensed for reuse under this Creative Commons licence