These sentences are correct!
Please note, you should not request additional work / site assistance from us, through the site assistant. Any additional work / site assistance should be requested in writting so that we can allocate our man hours accordingly. This will prevent any problems in the final bill at the end of the project.
Also, please note that we will do work on a verbal request, however you will need to give us the details of the work in writting as soon as possible.