Thanks everyone.
It's not really w working farm just an 80 acre property, so theres

We had a 'bush' wedding, had a marquee with a wooden floor, and people wore the same thing they would wear to a 'city' wedding. The marquee, floor and everything else was all brought in from offsite and set up- All we needed was a flat space big enough for the marquee.
That's what its going to be like from what I can gather, and the marquee is going to be not more than 30 meters from the main house.

Im a little worried my cousin and his DF are so laid back that they will say casual and then everyone will dress up except us.
Im going to give my uncle a call and ask him what the dress code is, and go for the slightly more dressed up variant of whatever he said.
As for DH, well if he chooses to ignore it he'll be the one underdressed and not me.