Can I use the key option on my Disco11 to lock/unlock /start instead of the alarm [ alarm annoying goes off at inconvenient times]. or is it possible to immobilize the alarm completely

 

The key works fine to lock, unlock, and start. I did not have a remote with my first truck for the first year that I owned it.
It will enable and disable the alarm though, so if you are having problems there, using the key may not solve the trouble you're having.
You may also want to change the door lock pin for an upgraded part before you rely exclusively on a mechanical locking process. The original pins have been known to fail. Carrs4x4 sells a good upgrade.

 

Same situation as mollusc. My fob "died", basically disintegrated due to the age of the truck (280K), as well as the ignition switch replaced with a D1 version so I have new keys, so I use the old key for doors, and the new D1 key for the ignition. I have replaced the door pin with the one from Carrs4x4
. Works great!