Note that some old copies of the manual require a correction to practical 2, page 16, line 4. The intercept should be:
If your copy does not have this, print out this page and insert it in your copy of the manual.
This Excel spreadsheet shows a graph of ln(rate) against 1/T with negative slope. You can download it and play with it to better understand Excel. (Your slope in experiment 2 should be positive since the graph is ln(td) against 1/T).
Values must be displayed with the appropriate number of significant figures. Excel sometimes rounds numbers to 1 figure. This is (almost) always wrong, and will lose you marks. This problem often occurs in the equations displayed on charts. Right click on the equation, select "Format data labels" from the menu and select "scientific" and "2 decimal places" or whatever is appropriate for a particular case.