This all sounds wrong but as a contractor working on site you have a right to see the " Asbestos register " . That will list any areas that contain asbestos. As a plumber you are unlikey to come into direct contact with asbestos unless you drill through insulating board, but then you should have been on an asbestos awaerness course.
When the removal compamy carry out a clearance they must have an indepent analyst do a clearance test and they will leave a " 4 stage clearance certificate " on site . You could ask to see this .
Also both the removal company and the analytical company should be listed on the ARCA web site found here .
http://www.arcaweb.org.uk/ARCA_members1024.asp