What sort of lock for this gate, and who would I get to fit it?

[RingUpRingRingDown]

To improve security I need some sort of lock on this gate. At the moment it has a latch (is that the right word?) on the inside/garden side of the gate, but it can't be opened or closed easily from the outside. So, if we go out through that gate we have to leave the gate open. I no longer feel happy doing that.

Ideally I would like some sort of lock that uses numbers rather than keys, like you get on an outdoor key safe. Simply be use children could then use it and not have to worry about taking keys out and about with them. But I don't know if that's possible or even very secure?

I'm thoroughly confused about what is possible with the existing gate set up and who would I get to install it - locksmith, carpenter?

The house is listed so we can't have anything too incongruous.

The pictures show the gate from the garden side (with latch) and the street side.

[PigletJohn]

I can't see a post or frame on the locking side. Is there one? Open the gate and show me.

Is it a house or a garden wall that the gate is set in?

I can't make out the latch. Is it a bolt?

What are the dimensions of the stile on the locking side? (the thick piece of wood that makes the side of the gate).

[RingUpRingRingDown]

Sorry, I'll take better photos and measure things tomorrow when it is light.

The gate has a garden wall on both sides. There's no wooden post, just a bit of metal set into the wall. Hopefully a better photo will make this clearer!