as far as i know they are pretty lax when you turn up, specially as they are so small and only have 3 staff they probably can't be bothered with the paperwork sorting out something this minor.
Saying that though, my xing was booked with 2 passengers neither right, my old cars reg and i just phoned up to pay the outstanding £25 and change these things while i was there, got a bit involved so checked out the web site mid discussion and the same xing, 9.10pm on the 16th was now £19 so i said forget it and bought the new cheap one on line. Saved £6 and a discussion. Bonus!