Could be that something has come adrift in the door itself or the solenoid responsible for locking that particular door is stuffed! I would start by removing the door card and seeing if there is anything obvious that's not attached (normally a rod)!
Hi All, I know this a relatively old thread.
I've just joined as I bought a Mark I (P Reg-1996)today and it has this problem, I just know it's gonna be a rod as said above.
I'll post back up once I've sorted it.
Not sure if I can post pic yet either, but if I can I'll drop one or two also incase it helps someone else in the future.
I race motorbikes, and have always been fairly 'get the spanners out and let's see what makes it tick' so hopefully I can contribute a fair bit once I know how this car works (always fancied a Laguna over the years, I came across this one quite by chance today)