For any future works and to avoid ambiguity it might be worth reviewing and perhaps amending any contract between yourself and the agent to include your preferences. Sound like a quick chat with the principle might be in order. Hopefully that's not the person you're already dealing with!
As an ex principal in a real estate agency for 40 + years I suggest you simply ring the Principal, tell him your problem and ask him to investigate it and sort it out, there will be an answer.
The plumber should provide a proper tax invoice detailing what has gone into the place. if you are not getting the answers setting out model number and preferably the serial number of the unit something is wrong
if you think the plumber is stiching you up then suggest to him you will ring the plumbers registration Board to sort it out. I guarantee he won't want you to do that.
Whilst owners would not agree, there is very little profit in Property Management for the time and effort that goes into it, most of which an owner never sees or appreciates. The only way to make it profitable as a Principal is to have good well paid staff who sort out problems not create them.
I suggest you put your concerns IN WRITING to the boss and see what happens.
If you are not satisfied then find someone else to look after your property BUT remember you only get what you pay for and if the agent is screwed into the ground on commission the chances are that is part of the problem. Pay peanuts get monkeys
It's a competitive industry but I have turned down lots of unprofitable business from owners who wanted to pay nothing who then went elsewhere and then came back in 12 months bleating about the poor service they were getting elsewhere etc etc etc, but please don't think I am saying this is the case here, just my observation from 40+ years and having the largest Property Management portfolio in my region, most built on happy Landlord referalls ( who beleives the buls**t in adverts anyway) when I sold my business.
Sometimes the only way people realise what is a fair rate to pay is to have a problem with the "cheapie" and then experience proper service from well paid staff who have a demanding Principal to understand the difference.
No longer my problems trying to explain that to people fortunately
B