I used a standard Jessops tank (similar to Paterson tank) taking two 35mm rolls. This is the same tank used for the second roll (APX) this morning. In both cases I used the rubber lid, so I cannot see how light could have got in part way thru exposure? It still would not explain why the undeveloped leader which I clipped off in the changing bag will also not clear, as this has not been touched by any chemistry.
If anyone can explain this one I'll be really impressed.