Mike Jenvey
Be careful with this...
You have quoted S.92 Being unfit for duty. There is no requirement under that section for a screening breath test to be conducted (although one may be given). The power to arrest is contained in S.97, and only requires reasonable suspicion that an offence is being committed or has been committed and the person still has drink or drugs in thier body. The evidence would have to be the same as for a person driving while unfit, which in the cases I had were basicly drunk.
The other section is S.93 which is having a BAC above the precribed limits, that would always be dealt with by breath test as the screening test and thus giving power to arrest.