without recreating your whole chart seperately, I can't tell if this formula works but I don't get any errors. The cell (data entry E4) I cannot enter a value into to check it against. So see if this works for you in B16...
=IF(ISNA(VLOOKUP(B8,Source!E:H,4,FALSE)),'Data Entry'!E4>=45,"Total hours exceed the Working Time Regulations. Please ensure that this casual has opted out by contacting The ,""")