Peugeot 406 flat battery, now charged can't start,lights flash immobiliser?

I have just got my first car a Peugeot 406 desiel P reg, unfortunately I have let the battery run flat and it wouldn't start...even by jumping it.

instead I found the lights flash for a short while and there is no life to the engine...

I charged the battery for 24 hours and still the same problem, there is life in the windows, wipers, lights, radio etc but nothing to the engine and these flashing headlights seems to be something to do with the problem...is this the immobiliser that has kicked in or something?

If so I am wondering what I can do? I've had a look at theis manual:

http://spidometrs.ru/s2/download/peugeot_manual.pdf

and looked to the bottom right of the wheel and only found a bunch of fuses? connectors? nothing to enter a code into or anything similar.

I have a red flashing (immobiliser?) light on the dash board above the radio and air controls, no other red or green lights as mentioned by others on this link:
http://www.howtomendit.com/answers.php?id=74265
So i'm a little stuck as clearly i'm not a grease monkey or car thief and have very little knowledge on what to try or do next. Can anyone help please, i am very keen to learn as clearly would be very valuable to know what to do or my options

email peter@illustrate-archaeology.com

many thanks in advance