Yep, that helps a lot. It's very unlikely to be the Cobra at fault here, unless the installers have managed to join both lock and unlock wires together, in which case it would never have worked.
The installation of an alarm, unless installed appalingly, will never stop the cars central locking from working. All the alarm does is puts a pulse down the factory system to activate the central locking relay as if you had pressed the button on the drivers door. It cannot stop that switch from working, (Again, unless installed by a blind version of Edward Scissorhands).
First place I would suggest looking is in the drivers door. Unplug the window/locking switch and check there is no water or condensation inside.
There have also been a couple of recent threads about the drivers door lock switch causing a problem. Having never experienced this personally, I can't make an informed suggestion, but worth a look.
It could also be the central locking control relay located behind/beside the glovebox.