Once the debt is registered at the Traffic Enforcement Centre, The council will sent you an Order for Recovery. The envelope should also contain the form to submit a Witness Statement. you can submit this Witness Statement stating that you did not receive the original PCN. TEC will cancel the Charge Certificate and Order for Recovery and the matter reverts to the PCN stage at which point you can pay it or appeal. You should get the discount option too.
If you're worried about mail delays, you can download the relevant form from the TEC website and when the amount outstanding increases by £10, you know TEC have registered the debt. You can find out by calling TEC every 7-10 days, or if you can see your PCN on the Bristol website, it will increase by £10 when the debt is registered.