Mondeo Car Door Not Locking

[FellowPazzini]

Had this problem before but on the front passenger door. Strangely a couple of weeks after a mechanic making a right mess of the inside sound protection stuff it was working fine without a fix.

It was noticed when the inside door handle lock (rear driver side) wouldn't lock, physically can't be pushed in. So I tested the lock by remote locking the car, all door lock except that 1. It's been like this for a week now (bit of a lazy git & skint) so I'm hoping this has happened to someone else who can advise me on what to do. Fingers crossed it's just a little tweak but knowing my luck it'll cost lots of money.

Any idea's?

Thanks