Is there anything I can do to challenge this?
I can't really think of anything.
The NIP seems to have been served in time from what you say. You don't seem to dispute that you were exceeding 20mph. That leaves really whether the limit is lawful and adequately signed.
You should be offered a course for that speed (if you have not done one for an offence that occurred in the three years prior to this one).
BTW, you may like to redact some of the information on your NIP, such as the reference number and passcode and your Reg. No.