My Basic problem seems to be that i cannot run a variable against a blank cell.
My set up as below:

Item 50cl 70cl 1.5cl per 25ml (< answer i'm looking for)
Rum £17.50
Cheese £80.35
Milk £10.50

Now individually i can work out these items. Rum would be 0.62 in the 25ml
column... but I cannot find how to create each sum in the total box, so when
i leave two of them blank it understand to only work out the one answer.

It must look something like this? Right?

Item 50cl 70cl 1.5cl per 25ml
Rum £17.50 =sum(b2/20) (C2/28) (D2/60)
Cheese £80.35 =sum(b3/20) (C3/28)
(D3/60)
Milk £10.50 =sum(b4/20)
(C4/28) (D4/60)


What am i missing in between each set of brackets to make the thing work
though?

The alignment is a bit out in your posting. Assume headings are in row
1, with Items in column A, 50 in cell B1, 70 in C1 and 1.5 in D1. In E2
type in this array formula*:

=SUM(IF(B2:D2<0,B2:D2/B$1:D$1*2.5,0))

*As this is an array formula, when you have entered it (or subsequently
edit it) you must use CTRL-SHIFT-ENTER instead of ENTER. If you do this
correctly, then Excel will wrap curly braces { } around the formula -
you must not type these yourself.

Copy the formula down column E for as many rows as you have items.

Hope this helps.

Pete