i am considering fitting an electric door opener to my garage but i am concerned about locking.
are the motor mech's sufficient to 'lock' the door without physically locking it by key.
any idea?

Here's what you need, thinking of fitting one myself:

http://www.youtube.com/watch?v=1C-Bxifv2Pk

thats not a bad idea however i could not rely on the door being locked-the device forgets to lock the back door to the house often!!!

BTW - Yes is the short answer to your question, they do lock it.

I just wet myself laughing. How funny is that.

the other thing about remote garage doors is that they can respond to signals from elsewhere and apparently open on their own when you aren't there.

I have had this happen a couple of times on rental properties where the garage door keeps mysteriously opening

I had the MK 1 version of that door opener but was forced to stop using it because it kept emitting a strange whining noise whenever the horn was pressed. It has also prevented MK 2 and 3 versions from operating in a satisfactory manner.

i complained to the manufacturer but was told that I was an idle buŁŁer and should do it myself.
I have had this reply from this manufacturer with regard to other matters, I now sometimes regret entering into a long term contract.......

Garage door openers are fine accept the up and over door can be forced at the bottom as the bottom locks are removed.
A better more expensive option is the roller door, which is harder to force open

mine seems to work fine.... but I live on a road where anyone trying to force the door would be very noticeable....