Mondeo boot button/lock?

I can only acces the boot of my P Reg Ford Mondeo by use of the ignition key, even when the engine is switched off and the car unlocked. I am sure that it should work by simply depressing the boot button but this has no effect. Do I need to replace a fuse or clean an electrical connection somewhere? I have not rushed to correct the fault as in some ways it is added security, but equally it can be inconvenient. Any help to identify and repair the fault would be much appreciated.