Unlike a normal door where you would fully recess the latch into the door and it would close against a frame with a rebated double door like ours the latch actually needs to sit partly out from being fully inside the door.
Fitting a mortice lock to a rebated door.
Position a mortice deadlock about halfway up the door and fit a night latch on your front door one third of the way down.
Mortice latch or tubular door latch showing the latch casing the catch and door handle spindle insert.
If you want to fit one yourself you ll need to be capable of woodworking as well as using the tools required to fit the lock.
When the door needs to be opened a door handle or sometimes a knob is turned which in turn is attached to the latch via a door bar or door spindle that runs through the latch.
If you only fit one lock go for an automatically dead locking night latch made to bs3621.
Easy to follow instructional video on how to install a mortice lock in rebated double timber doors.
The second important part of this how to is to buy a rebate kit.
To install this lock see section 4 below.