This is a lovely old mortice lock with a broken spring inside. This spring normally returns the latch to spring it back out upon being pressed inwards.
We carry a few different sizes of sprung steel springs for replacing broken springs in these locks and it can be quickly replaced.
The most difficult part of this job is usually getting the old lock out of the door to be able to open it and replace the spring inside.
Some of these locks with this type of spring are mounted on the face of the door and called a rim lock – others, like this one are mounted or morticed in to the door. Either way it’s the same type of spring – although there are a few different sizes of spring inside some of these locks.