common sense would be the employer had more people working the shift so that the job was done on time for when everyone's shifts had ended mate.
If you all laze about and dont finish then you get warnings, nuff said.
You're well in your rights, so what if the other people would take longer to finish and go home, they are not being part of the team by not standing up along with you, YOU are in the right.
If you havnt finished all the work you either all need a warning for not being efficient enough OR the company needs to have more people working the shift, or just pay you for the OT.
Hate people like that at workplaces, iv always left on time without fail, im not working for free. Your employer the type that demands everyone turns up 15mins early etc? Had a job where i got warned by a supervisor because when my shift started in the call centre I couldnt take any calls, as my computer was logging on(it would be painfully slow @ 5pm due to evening staff all logging on at once), i told her where to go when she said I should be there 15mins early so at 5pm I was sat, ready to take calls all fully logged in.
Im sorry pet but i dont get paid for it, and its not my problem if the network isnt fast enough to take the strain of the people all logging on at once at those times

it took me till 6pm one time to log on, first into windows, then through the web interface into another program via citrix