Interesting experience from Patrick who emailed me with the following:
"Had to inform you of a peculiar incident that occurred to me on the 13th July on the Metropolitan line. At Finchley Road, a lady dressed shall we say rather casually (jeans & t-shirt) hopped on the train and proceeded to ask passengers for tickets. Nothing unusual in that you might say, though it's my first time I've been asked for a ticket in all the years riding the Tube.
The lady was going through the carriage swinging a Primark bag (!) in one hand and a ticket machine in the other. Everybody on the train seemed to think it was a bit bizarre and questioned whether this inspector was for real. Maybe plain clothes inspectors dressed casually, in this case very casually, disguised as ordinary passengers carrying shopping bags is the latest innovation by TfL!"
Occasionally on the District Line I get asked to show a ticket, but it's pretty rare and in those cases, the people asking look pretty obviously TfL dressed. I know there are a number of plain clothes inspectors hanging around stations, but I've never heard of them on the London Underground carriages themselves.
I suppose if she had a machine checking tickets, it's pretty hard to fake that & perhaps the Primark bag would have added to the "undercover" look even more.