You would have to agree a payment from Cadburys or Walkers or whoever as full and final settlement, and formally waive the right to make a further claim in the future. The statutory rights thing means if you return them, we'll replace them/give a refund, but it's later discovered that you contracted some terminal disease as a result of eating that packet of crisps, you still have the right in law to go after the manufacturer.
I think.