Thanks Gunsmoke, I 
think that all makes sense to me. However I'm not sure that it's totally heading in the direction I'm looking.
What I would like, for example is something like this - 
3 checkboxes, one £1.00, 2nd one £5.00, both selected, and when 'submit' is pressed this goes to the cart with a total of £6.00 (as working)
OK, - 3 checkboxes, one £1.00, 2nd one £5.00, 3rd one - I would like a
 minus function - say £2.00, and when 
all three are selected, so now when submit is pressed this goes to the cart with a total of £4.00
Hope that makes sense ??
Dave
            
                                    
              'A computer lets you make more mistakes faster than any other invention with the possible exceptions of handguns and Tequila.'