Shit no this rings a bell. Steve (if that's your real name, sorry if it isn't!) - did anyone in your road get burgled on the same day by any chance?
It's an odd question - last year when I was at my ex's in Hackney some people came knocking asking if we wanted to buy something (I think it was fish too). Later on that day one of her neighbours was burgled. I don't think they ever found who did it, but perhaps they could have been scouting the area to see if houses were empty, probably knowing that no one would buy fresh fish off someone who just knocked on their door.