Not sure if this is insanely obvious but i can't seem to find anything for it in any of the dialogue tutorials.

I'm trying to set up an NPC to talk to my character as i approach it (the scene is essentially you approaching a murder scene and someone talking to you when you try to get near the body). I want it to be like how a guard stops you automatically while you have a bounty, or when Brynjolf first approaches you in Riften, but checking the generic guard dialogue for such events i can't find what sets the guard to talk to you, rather than you talking to the guard.

Could somebody tell me how to set this please?

You should give your actor a force greet package. Put your npc in an alias and add a force greet package to that alias. When creating your force greet package just use the forcegreet package template but fill the topic with your custom topic.