Are you trying to develop a occupany calculation for the current day, as a running total, or are you trying to only provide the latest occupancy data for the last interval or 5 minute period?
 
Ed
 

Well your 3 tables for Skill groups for Available time are:
Skill_Group_Real_Time.AvailTimeTo5 (gives you total available time for the last 5 mins)
Skill_Group_Half_Hour.AvailTimeToHalf (gives you total available time for a given 30 min period, can sum this historical column for the day but will only be as up to date as the last 30 minute interval)
Skill_Group_Interval.AvailTime (like the previous value, this gives you total available time
for a given interval period, can sum this historical column for the day
but will only be as up to date as the last interval)
 
So with that in mind, you can do a running total for the day using subqueries but it will not necessarily be realtime as it will only update every 15 or 30 minutes (depending on which table you decide to use). The rolling 5 minute window is your only option for a true realtime occupancy calculation (ie occupancy for the last 5 minutes).
Cheers,
Nathan

Thank you Ed & Nathan!
I wasn't sure if I was missing a trick - we'll go with the Real Time rolling 5 mins and the historical figure that updates every 30 mins!