OP,
google - independent damp surveyors or independent damp and timber surveyors.
When you've located one or two in your area come back here with their names, & I'll tell you whether they are truly independent and qualified.
Possibly, there are neighbours who are also interested in getting a definitive diagnosis of the causes of damp in your properties - perhaps a few of you could hire the independent surveyor as a job lot, so to speak?
A proper surveyor will not only pin down the cause or causes but also say whats to be done for what kind of estimated costs. ie they will prepare a costed schedule of works.
Some surveyors will also oversee the undertaking contractor's work.