I hope I've understood this correctly, Lozzy. If you just want Column F to contain a running total of expenses, irrespectice of the code in column C then:
Assuming that you have titles in row 1 and the data starts in row 2 then in F2 enter the formula:
and press Enter. Then hover the cursor over the bottom right of cell F2 until it turns into a +. Hold down the left mouse button and drag down as far as required. This will copy the formula down, for example in F4 it should read
and give a running total.