time sheet formula

hi there i am trying to build a calculator for the UK working time directive it requires workers to work no more than 48 hrs a week averaged over a 17 week period as drivers hours vary from day-day week - week i have made a weekly time sheet style spreadsheet which is fine until i get to

if one week they work 56 hrs = 8 hrs above 48 hrs
next week 48 hrs so that is equal to the weekly
3rd week 42 hrs so they have worked less than 48 hrs
etc etc
the week starts sun finishes sat if a day is not used for any reason hol sick etc the value is counted as 00:00 as i have done the hrs as a time formula

eg wk 1 total hrs worked 56 = 8 hrs above 48 so 48 -56 = 8
but what if hrs are less 42 hrs worked so 48-42 = +6
i can do the formula one way or the other but not both

i'm ok with basic formulas but this has me stumped
thaks in advance if anyone can help