Gill, what is doing your job properly? This is a vague and ambiquous expression...
You can try and do the job as best as you can following procedure and all that but the employer/management do have clauses that they can invoke to get round some perceived ways of doing things.
The idea of cutting-off is not as easy as some people here thinks - for starters, the Union (if I remember correctly) has aready agreed that we can be 30 minutes over depending on circumstances... Furthermore, there are times that we do not actually do the full hours that we are paid. Our employer can force us to return to base/depot to complete that time - hassle, I know; and people can then work at a slower pace (good!)...
Anyways, postmen rarely ever stick together on issues, so management can get round us and/or take advantage of divisions.
Some offices are tough, so managers are just as confrontational as some posties and these are hard times; continually cutting-off in some office might lead to aggravation and possibly conduct or even the sack. And the Union might not even be able to defend some people and/or their actions.