I'm looking for pointers before I get started developing one from scratch, I do not have, or plan to spend money on other peoples scripts or products to import into my project which means I can neither see how they do it, nor use their systems directly. As such, this will be entirely hand written to match what I need.

The specifications are as follows:

• Unique Actors that prompt different conversations, whether that be an NPC conversation, or the interaction with a work station and selecting options.
• Branching conversation paths (and therefore a method of storing variables to short and long term store flags for different states)
• Localised scripts. 
  • I do not want to have 3 or 4 different scripts on every object just for this, it would be optimal in my opinion to have a single dialog manager script, but i will be happy if i have to include an "actor" script on the different NPCs along with that.
• UI (arguably the easiest of the requirements that will sort itself out when I've got everything else working over time)

Is there anything obvious I'm missing about functional requirements, non-functional requirements? What is the best place to get started?

There are some tools out in the wild that provide exactly what you need but thei are unfortunately paid but most of them have a free demo period so try to see what you get to complete your list.

Arcwave

Articy Draft

Also there is a GDC Talk you can watch

https://www.youtube.com/watch?v=XPPtLNkVPWY

Perfect, thank you. will watch the GDC first and then follow that up by examining both of those products.

Correct me if I'm wrong and have missed something but both of those products and the video seem to be based around game design, documentation, and flowchart management etc with relation to systems like dialogue, story and game events. Is there actually dialogue managers, i.e. actual components of the game itself for dialogue management, just like a character controller and camera controller, in these products or have I correctly assessed them as design tools.

If these tools do indeed have working dialogue managers, could you direct me as to how to find them, as I must have missed them.

Additionally, though it doesnt matter extensively since the languages are similar and i can more or less read both i have realised that i tagged this topic incorrectly as c++ instead of c# as i usually do via misclick. Just thought I'd make mention of it in case it helps.

Yes, these tools provide game design capabilities. Your request is a little bit of both, Branching and localisation is handled along together with many other game design features by those tools, the Unity dialog system is then provided by their Unity Plugins. Both Arcwave and Articy offer a Unity Plugin and demo projects that allow to import their content into Unity and provide the dialog systems to use the content