Mortice lock problem

[Sanjay29]

The front door lock is a little temperamental. It locks and unlocks very easily from inside the house. When locking and unlocking from the outside it is much more difficult and feels as if the key might break.
See pics.
Any advice please. Thanks.
Will wd40 help or not. ?
Have not tried it.
Thanks again.

 

[blup]

Its more worn from the outside because of greater use there.

Dry lubricants like graphite powder are best for locks, but it might be time for a new one.