I've been trying to get new dialogue to show up on new NPC's I've added to my mod.

Based on the guide from the CS wiki, I have made a new quest, given it a priority of 75 and added the Getplayablerace script to the quest conditions. I then went to topics, added GREETING in (from the list), created a line of dialogue under this topic for one of the NPC's in my mod and set the GetID condition to this NPC and set it to 'say once' (I only want him to say this once before returning to the usual greetings). I then created another topic about my mod, created a line of dialogue for it (intended to be used with the same NPC) and used the same condition, also setting it to the NPC I wanted to say this line of dialogue.

However, these new bits of dialogue won't show up on the filtered dialogue pop up for this NPC with all quest set and won't show up in game. Futhermore, on the filtered dialogue window, if I add this quest to the NPC's filter no topics show up at all. So I know I've done something wrong. Anyone got any idea what?

If it just that one NPC you want to say the thing, you could try to set in the quest conditions a condition GetIsID YourNPC != 0 instead of the GetPlayableRace thingy... If it doesn't work, check that the dialogue condition really is ... == 1 instead of ... != 1.