I have an audition coming up in about 5 weeks, and I really don't know what to play for it. It's not really formal or anything, and there is no pianist there. Does that mean I have to play something for flute alone, or can I do an accompanied piece just minus the piano? It's for an orchestra and the players are expected to be at least grade 8.Has anyone got any suggestions for pieces of this sort of level that I could play? I've got plenty of time to learn something new, and would enjoy the challenge

Well the two obvious pieces of Grade 8 standard are the well known solo flute pices, 'Syrinx' by Debussy, and 'Danse de la Chevre' by Honegger.
Im not a flute player , but enjoy both of them.

I never had an audition, so I can't answer the question if you have to play Solo-pieces. But I have some suggestions for pieces for solo-pieces.

Marin Marais - le folies d'espagne (it's about Grade 6-8 Standard)
J. S. Bach - Partita in a minor
C. Ph. E. Bach - Sonate in a minor
Varèse - Density 21.5
Jolivet - Cinq Incantations
Ibert - Pièce pour Flute seule
Bozza - Image
Fukushima - MEI
Berio - Sequenza
Ferneyhough - Cassandra's Dream

If you want some more suggestions, just say a word.

If you decide to play a piece that is normally to be played with piano or orchestra, it must be one, where you don't have breaks in between, that should be filled by piano or orchestra.