I'm struggling with how to diagram 'up at'. Is this a two word or complex preposition or something else?
Treat it as a single preposition for the purposes of diagramming.
There are examples of diagrammers treating these as a single preposition. For example, in this model "according to" is written on the same branch without further distinction. The author, Eugene R. Moutoux, notes:
If you counted, you may have missed the seventh preposition, according to, a compound preposition. Some other compound prepositions are because of, on account of, except for, out of, instead of, in spite of, and next to.
In another diagram, Moutoux depicts "on account of" on a single branch and calls it a phrasal preposition.