mdj101 wrote:
I have the formula =SUM(D2*(20/100)) which gives me the discount
value of cell D2 i.e if D2 = £10 then the SUM = £2
But I want the formula to work out the total minus the sum ie.
£10 - 20% = £8

There are usually many ways to accomplish the same thing.
Perhaps the simplest for you to understand is to formulate it
exactly as you described, to wit one of the following:

=D2 - D2*20/100
=D2 - D2*20%

Note: There is no reason to use SUM() in this formula.
I am curious: why did you do it in the first place?

Also note: You might want to add some appropriate rounding.
What is "appropriate" depends on your goals and monetary
system. For example, one of the following (for US dollars):

=ROUND(D2 - D2*20%, 2)
=D2 - ROUND(D2*20%, 2)