I am trying to find out myself on Google as well but it seems fairly complicated.
My gf has to clock in at work 5 minutes before her shift starts, to allow her time to get to her house (she works with autistic people), take off her jacket and bag etc. The reality is it takes her 30 seconds to get to the house, and she is good to work within 60 seconds or so.
If she clocks in at say 6.57, for a shift that starts at 7, she is losing 15 minutes pay for being late! How the hell is this fair? I am going to tell her to not work the 15 minutes in future if they insist on it. Most of the time, she is really early (like 20-30 minutes) because she walks to work and leaves plenty of time. For some shifts, I take her and the occasions she has been late is because we have hit unusually heavy traffic or something like that.
There has only been one occasion when she was after the actual start time - around 2.50pm, for a 2.30pm start, due to huge tailbacks on the A1.
I hate employers holding people to ransom. They don't pay you extra money here and there, so why should they expect that you work for free? They don't pay extra if you clock out five minutes late.
On a side note, my boss worked 500 unpaid hours of overtime last year. 500 hours. I worked it out to be around 16k of his time based on an estimated salary!