Simple formula not calculating correctly

Hi,
I have a very large spreadsheet being used for a business model. We have 12 sheets in the work book and 11 of them work fine. In the 12th, which has been formatted identically to the others, a simple sum formula (i.e. =sum(A1:A100)) is returning a false value. This is occuring in several columns. When I select the cells I get the correct sum total on the bar at the bottom of the screen, but the formula returns an incorrect total.

Any suggestions gratefully accepted as many hours have been invested in this work book!

Thanks