Did you try from the inside? Also try unlocking and locking a few times with the buttons on the dash. I have the same problem on my Disco 3 on very hot days (a few last summer) and very cold days (during the winter). Is it hot where you are?
Discos & Door central locking are a perenial problem on all discos apparently... Just about to replace the locks on my disco 2 front doors...
