Biggles...I work for 2 operators and have had to be tested on separate occasions for each operation. Apparently the results cannot be carried over to the other operator because the test needs to be done on commencement of employement (in my case there were only a few months in between) I think this is because the test is meant to be done circumstantially...just incase you decide to acquire a drug habit in the interim!
On both occasions my employer has paid for it so no skin off my nose!