I know this is old stuff but perspectives are always good.
Yes, I have sat them down & the response I go was "We can finish at 5pm if we want surely?"
I used to work part-time at a branch of a national chain of catering and grocery wholesaler/cash and carry business. While, yes, we could go at our contractually scheduled leaving time, this was almost unheard of. We nearly always ended up doing a few minutes to half an hour overtime as we very rarely were ready to leave by close of business as the shop closing to customers did not generally coincide with being packed away and safe to leave overnight.
This was a matter of discussion among staff. Our union informed us that we could choose to leave at our contractual leaving time (which was fair, as, that extra time not being contractual, we were not guaranteed those hours so the deal was slightly one-sided if we could be forced to work overtime). In practice, though, we were paid (some of us were even paid overtime rates) overtime for those extra minutes and, unless we had prior engagements, we understood that what needed doing had to be done before we could reasonably leave.
While the management tried to pretend they could force us to stay, this never came to a head while I was there. Obviously the management was not going to set a precedent by pulling someone through a disciplinary meeting and trying to give them a verbal or written warning for the offence of 'leaving on time' as they would have lost. What tended to happen was that when we came on shift, if we had to leave dead on time, we'd tell whoever was in charge that day and they couldn't really say anything. The only person who really got told off as such was someone who left an important task unfinished without having informed anyone in charge that the task was incomplete.
If you are paying 15p above the statutory minimum wage like my former employer, then regularly expecting employees to work (paid) overtime may be unreasonable, especially in wholesale where the work is more predictable than in plumbing (the overtime could have been factored in when they took us on and we could have officially worked slightly longer weeks). But in a craft trade such as plumbing, I'd expect a craftman to want to plan jobs to be able to leave them in a fit state at the close of the day. Paid for the extra hours, okay, but, to a point, they could be expected to be obliged to do them, and this could quite reasonably be made contractual.
