For this particular thread and bailiff action, the only way to get the matter reverted to the PCN stage is to submit an Out-of-Time statement.
Depending on what Act the PCN was served under, this will be a Witness Statement or a Statutory Declaration, but we cannot tell you anything more because you have told us virtually nothing about what caused the PCN to be served. For parking under the Traffic Management Act, it is a Witness Statement, and for moving traffic PCNs, it is the London Local Authorities annd Transport for London Act 2003.
As you are out-of-time, you need to submit two forms in either case: -
TMA 2004 - Forms TE7 and TE9
LLA & TfL 2003 - Forms PE2 and PE3 (must be witnessed by solicitor, or at a county court)
Traffic Enforcement Centre forms, including the form to challenge an unpaid penalty charge notice.
So why two forms ? One form is the actual declaration, and the other seeks permission to submit out-of-time and a reason must be given. Either declaration is passed on by TEC to the enforcing authority who can, and invariably will object to it, following which TEC reject the submission. When the forms are processed by TEC and the authority are informed, bailiff action must cease until a decision is made. The only problem with this is that TEC now take a long time to process forms.At one time it was within 24 hours, but now days elapse.This give bailiffs plenty of time to carry on enforcing, incl a visit with its £235 fee, (which is statutory BTW).
I would reiterate that the website I gave you is your best option in getting assistance. If it were me, I'd now pay the bailiff, because this has no effect on whether your statutory submission is accepted or not.