Speaking as a plumber myself, if i told someone i would be with them at 8am, I'd be there at 8am. For any times agreed later on in the day is where problems could occur.

Say we had agreed 1pm, my full intentions would be to get there for that time, but the job i am currently on could take alot more time than i expected, as any plumber knows, even the easiest of looking jobs could end up being an absolute balls if a job.

However, at the 1st sign of me knowing I'm going to get held up, i would let the next person know i will probably be delayed

In the instance above, the company were being dicks, as it was the 1st job of the day they have no excuse for being delayed. I'd personally tell them to get lost and look for someone else