I can't override my renault megane immobiliser using the central locking bu?

My renault megane will not start, the immobiliser will not switch off, I can unlock & lock the car but not start it. This problem occured after I left the lights on and the battery went flat. I have obtained the code needed to override, however the central locking does not respond (when I press it the flashing does not slow down, hence I can't insert the code) I have tried pressing the accelerator pedal & pressing in the end of the window wiper wand with no avail, do you know of any other way of inserting the code and hence overriding the immobiliser?
Many thanks