If U2 is the opening time and T2 the closing time then you have those two the wrong way round in your formula (and E2 and F2 on the attachment). That won't be a problem when both dates are the same but over longer periods you will get incorrect results, so formula in attachment should be as follows:

=(NETWORKDAYS(Table1[[#This Row],[Date/Time Received]],Table1[[#This Row],[Date/Time Resolved]])-1)*($F$2-$E$2)+MOD(Table1[[#This Row],[Date/Time Resolved]],1)-MOD(Table1[[#This Row],[Date/Time Received]],1)

That will count all the hours between 08:30 and 17:00 on your attachment........but as you say it doesn't work if the dates/times are outside work hours. Assuming that only the received time can be outside work hours you can change to this version to calculate correctly in all cases:

=(NETWORKDAYS(Table1[[#This Row],[Date/Time Received]],Table1[[#This Row],[Date/Time Resolved]])-1)*($F$2-$E$2)+MOD(Table1[[#This Row],[Date/Time Resolved]],1)-MEDIAN(MOD(Table1[[#This Row],[Date/Time Received]],1)*NETWORKDAYS(Table1[[#This Row],[Date/Time Received]],Table1[[#This Row],[Date/Time Received]]),E$2,F$2)