Central locking problems

I need your help. My new(well new to me ) 1999 T Octavia estate has a locking problem its a SLX model with cental locking not remote (is this normal). it appears to have had a previous alarm fitted by a dash mounted LED(can someone confirm this). but i am unsure,

The problem is that when unlocking the car, most of the time it unlocks, but when locking it, it tends to lock the drivers door only, i can eventualy get it to lock but can take a while, and i do get annoyed so end up locking it from the inside the doors

many thanks

Diesel dawg :confused: