I have placed a two hour show (mp3) in the clock schedule but at the appropriate time the software doesn't find and play it
You need to ensure the software is in Semi-Auto mode, other the clock will not schedule automatically. If you want to schedule manually, go to Manage > Playout Log, select the time you want to schedule and click Schedule and OK.
at 21.30 last night I scheduled a one hour show in the Clock schedule for 22.00hrs. I then selected semi auto thinking that the software would then wait until 22.00hrs but it immediately started a random show from the tracks list. Is this because the software HAS to have something to play in auto.. or have I got a setting incorrect
In Semi-Auto/Auto Mode PlayIt Live will want to ensure that something is scheduled for the current hour. You could schedule a 60 minute Break Note for the 21:00 hour which would ensure that silence is played out and on your 22:00 clock make sure that it is started with a Hard Fixed Time Marker at 00:00 to ensure that your 22:00 clock starts dead on time.
Alternatively, you could just ignore clocks altogether and use a Scheduled Event to insert a track when it comes to 22:00 and then Play Next: