Negative Prices on Price Lists

I am trying to add a product line that will be a discount line.  This is a fixed value of say £10 when customer meets certain criteria.

I do not need CRM to make the judgement on whether or not to apply the discount, I just need to have a line that states:

Discount Because of XYZ   1  -£10

I can create a price of £0 and then use the price override function to amend to -£10 but cannot find a way to include this is a selectable price list.

I have already tried to edit the field definition for pricelist items to no avail.

Any help would be appreciated.

Kind regards

David