This is an opportunity to show the Septics what style is… 😉
No comedy ties or cummerbunds. Self-tie tie, obviously.
No colour other than black, except for the lining. No Braid or velvet.
No wing collars except with white tie and tails.
Ideally the jacket should look well-used, as though you wear it regularly. Try to get a few custard and wine splashes on it.
You can divert from these rules if you are known as a “character” and have the personality to carry it off. Your dress needs to be more understated than you.
If in doubt, ask yourself what Jeeves would do… 🙂