On modern cars they try and make it as hard as possible for thieves, if you can't access the release mechanism, only thing I can suggest is with the panel off spray loads of WD40 around the mechanism, then if you are outside with the window down try and hold the inside door release lever in the open position whilst working the unlock key button and pulling/pushing on the door to take the pressure off the catch
I know it sounds daft but I have had it work sometimes.
Apart from that all you can do is look at a new lock and try and work out a way of releasing it from that.
Modern central locking door locks are all built to be as secure as possible.