Im not sure I am replying properly- using quote? But yes, good call. I assumed it was a spot of corrosion or from an acid test. It feels smooth. Ill have a better look tomorrw with more light. I have tried wwd40.
But the camera is intriguing. My son also has an old victorian house with some suspicious blocked up bits and holes. I watched the people who installed the stove use one on the chimney breast, which was pretty scary! I may get one. We all fantasise about hidden treasure in these houses.