ML270 with a dead battery, manual key not turning to unlock driver door
 

Hi everyone, I'm new to the site and looking for the magic spell to unlock a 2002 ML270CDi please?
My brother gave me the car for when my Disco 2 finally gives up the ghost which will be very shortly. I've had it sat on my drive and have been starting it and running the engine for half an hour periodically. With one thing and another I haven't had chance to do so for 4-5 weeks and now the battery is dead. The manual key will not turn in the drivers door lock now either (it has been doing) not one way or the other. Any suggestions greatly appreciated. Thank you.