Well, the explanation from ATOC seemed to make sense, but in fact it was incorrect. Yesterday I made another journey from Crayford to Norwood Junction. Just for variety I tried travelling via Victoria on the outward journey because engineering works meant that the trains from Dartford were diverted via Sidcup. It all worked correctly this time, until I came to go home. Returning via London Bridge I made sure that I used different gatelines to exit and enter via the concourse. What happened? I was charged a double journey again. I’m beginning not to like this Oyster system. Another email to Barry and we’ll see what they say this time. I’m also writing to the Oyster helpline to report the issue.