If I write to the authority to challenge a PCN, will I still be able to pay the 50% discounted rate if unsuccessful?

Authorities are only obliged to accept the reduced penalty amount when the payment is received by them within the discount period (usually within 14 days). This is stated on the face of the PCN itself.

Writing to the enforcement authority or lodging an appeal, however promptly, does not freeze the discount. The full penalty amount applies, although some enforcement authorities will offer the motorist an extended discount period when representations have been rejected. If a subsequent appeal is lodged and refused by the adjudicator, the appellant has 28 days to pay the penalty at the full rate. The penalty will not increase during the appeal process, but it is the full charge that is frozen, not the reduced, discount amount.