Are you in the union? Because getting a union rep to talk to personnel/dept. manager can get better results than having to do it all yourself.
What do you mean, you don't get paid overtime? Meaning you don't get overtime rates for overtime you do during the week? Because you can probably expect that only working part-time, overtime rates are generally for full-timers taking on extra hours. Or do you mean Sunday premiums?
Re. finishing late, I'm pretty sure it's illegal for you still to be working after you've clocked out. Health and safety reasons, if you're not officially there then you're not insured. If they try to keep you past the end of your shift without extra pay appearing in your wages, firmly and politely say that you need to leave, now. Otherwise they're breaking the law and you're being ripped off.
Stick at it with the job hunting, have you been in the job centre? They're pretty good, have a computer search facility where you can pretty much key in everything you want to find (unfortunately I don't think there's a 'no nightmare managers' option). You sound like you really need to get out of this job. Good luck chuck.