Just remember that the council have a duty of acting fairly. On the face of it, chasing a PCN 5 years after it was served seems to me to be failing in that duty.
However, as you've just received this OfR, you need to immediatly submit either a Witness Statement, or a Statutory Declaration, to TEC. Which one depends on what the original PCN was served for.
So please tell us more, (a lot more !!).