I would think that there is nothing stopping you from claiming for a lower amount while reserving the right to pursue the additional difference subsequently, however, I’d be quite sure that you’d be entitled by general common law to write off the additional £250 and forgive that portion of the debt prior to the issuance of court proceedings. Anyone has the right at any time to forgive all or part of a unilateral debt. You don’t need a law to give you permission to forgo or forfeit a right that you don’t wish to assert.
Law Stack Exchange is for educational purposes only and is not a substitute for individualized advice from a qualified legal practitioner. Communications on Law Stack Exchange are not privileged communications and do not create an attorney-client relationship.