The middle one of the three pics above is the best I can get. There's literally nothing visible/accessible.Post a picture of it.
Andy
If you look where the white limescale mark is on the LH corner of the bit of moulding in the middle, if I follow that up to the corner it seems to be coming from there. It feels 'wrong' to the touch.
Filling back to max now in the hope of tracing it....