What to do if your bike is stolen

I'm not one to jump to conclusions, but I saw something today that seemed suspicious to me. At Wandsworth Common station there was a scruffy looking scowly kid standing with an blacked out Leader-style build (possibly Visp?) with his jacket hung over the drops. It had a red chain or something similar securing the saddle to the frame. The bit that made me curious was that it had LOOKs on it, but he was wearing his Nike Air Forces. Anyone had something like this go missing in that area or nearby? I went a bit Donal Macintyre and took a quick snap on my phone, but it isn't great. Hopefully it was genuinely his..

dont mean to get all PC on you since i know you had good intentions. but ive been on this forum what 4 years or something selling bits here and there. Air max's happen to be my fave trainers i always wear lol and I could easily be seen as someone "who doesnt look like they know/should know about fgss type bikes"