Cotton wool would leave tiny almost invisible hairline marks on a mirrored proof field - you really do have to treat those with the proverbial kid gloves. The more mirrored the surface, the more it's likely to show any contact. Think of a camera lens, then think of an ordinary bottle - the former shows marks much more easily. Good point, I will leave it alone, I can almost hear you wincing at the thought of me trying to clean this coin with soap and water or a cotton bud, looking at it again I can live with the mark weighed up against the risk of damage. Thank you for your patience and the sensible advice.