central locking?

03 plate rover 75 the central locking will not lock the doors. If I lock the doors by hand the fob will unlock them however it now unlocks all 4 doors at the first click of the fob button where before I had to click the button twice in order to unlock the passenger doors.
There is a warning light on the dash saying that there is either door,bonnet or boot open. This stays on when all are properley closed. Now every time I press he lock button the alarm beeps.
Have also tried standing nearthe car and pressein the lock button repeatedly, no joy.
Someone suggested a micro switch in the drivers door as the most likley problem as that door gets the most use. Where would I find this switch? Is it part of the door lock?
Any help would be apreciated.
John