I've been caught out a few times. I've also managed to avoid some fines when other people were done, by luck. There seems to be a lot of use of numberplate cameras these days rather than employing someone to check the cars. You really have to check the charges and times. One near here had a charge for all day on Sunday, but it turned out that was only until 6pm and then you needed to pay again if your stay was from 5-7. I think that if you paid something then that ought to be enough. It has to be well regulated and companies fined if they are out of control. I expect they lobby against that.