I got a new client in the last few weeks who had been paying an employee but hadn't registered. I did it for him (I called HMRC rather than doing it online, not sure if that makes a difference) and the paperwork came through within a week. There are no fines at the moment for late registration or FPS submission. If you have payroll software you can submit the FPS without being an agent so that saves a bit of time. If you get organised tomorrow you will probably be in time. Good luck.