The water bill is way too high for this tyoe of property
If you contact a good local plumber, and your management company should know somebody, the first thing they can do is a fairly easy reverse pressure test which will tell if the pressue on the system is being maintained and be able to find if the leak is internally or externally and even the section where the leak is
Yiu may have a leaky swimming pool as most pools have auto fill systems that automatically top the pool up, easy test is to turn this off in the control box and see if the pool level drops
Register a formal complaint, using the from, against Agualest over the bills which then require then to do a proper investigation on the meter
I annot recommend anybody but a couple of companies spoken about on other forums, Woolyspanners and Drips and Leaks