UPVC Front Door lock

I have a UPVC Front Door that opens from the out side via the handle unless the door is locked by the key.

I wish to change the arrangement so that the door is only opened from the outside by the key.

My concern is that if the key has not been turned to lock any body can enter the house from outside by just turning the handle.

What do I have to do to the mechanism to give desired results or do I have to buy a new door lock mechanism?
Can any one help with detailed instructions on what to do?
Plonker

it all depends on the lock you have fitted at the moment,unless you know the exact make and model or can remove it from the door and post a pic its impossable to tell exactly what you need to do.
in simple terms though,if the lock has 2 holes for the handle spindle (one above the other) then its usualy just a case of the correct type of furniture and an extra hole in the door.
if it only has the one handle hole in the lockcase then a new mech is required,and thats if your particular make does a model with the function you require.
expect to pay £25-£35 for handle furniture and £60 upwards for a new mech.