For hourly jobs, the minimum charge is generally one hour (apart from cleaning which is two hours and man & van which is an hour and a half). For fixed rate jobs, the minimum charge is the fixed rate of the job. Simple.
For some jobs, the professional might diagnose the problem, but you may decide not to have the work done. In those cases, you'll still need to pay the minimum charge for the job as they have taken the time to come help you.