Player has been told to meet somebody but must come alone. If player arrives with a follower the person he is going to meet won't appear. I already have 2 conditions for the person to appear, specific day and a quest stage - all this works. I need to add a condition, somewhere, that if the player enters the cell or better still is outside the door to the cell (this is where the game day and quest stage is checked via a triggerbox) that he is warned if he has a follower to make them wait or dismiss them, making them wait outside would be more natural. I'm guessing that with my existing conditions on the triggerbox I need to add another that checks if follower is waiting, how do I do this? In the worst case player will be informed to dismiss their follower. Help appreciated.

Can you just block the door activation after the player enters?

Or attach a cloak to the player after he enters the house with a script to detect people. Have your guy to meet initially disabled so he can't be hit by the cloak. If Event OnEffectStart is triggered, then send some way to communicate that another NPC/folllower was detected. Otherwise if this isn't triggered after a second or two, proceed with the meeting and enable your guy to meet.