Ford Focus - Boot keeps opening when ignition turned on?

We have a 51 Focus and it goes through periodic phases where the boot pops open every time you turn the ignition on and remains un-catched until you turn the engine off, when it will stay locked. Will do it for days or weeks, then stops for no apparent reason for months, until one day it suddenly does it again. Garage thought it might relate to the button on the dashboard and removed it, but that hasn't made any difference to the problem. Now thinks it relates to electrics and says cost to investigate it might be high.

Does anyone know how to sort this out or at least where the problem might be?

Thanks

Della
October 2008