I am looking for people's suggestions on how they paint wood.
Specifically I am talking catapults/trebuchets etc.

In the past I have used scorched brown, brown wash then highlight, but it doesn't feel right.

I'm guessing it should be more greyish ? What sort of wood would be cut for siege equipment, if freshly cut what colour is it, if they are older machines would the weathered wood be a different colour. Plain dark brown - hmm

Grateful for any advice !!

Yes, weathered wood is silvery-greyish, and fresh-cut lumber is more yellow-brown or red-brown, depending on the type of wood.

To get a good weather-exposed look, start with a lighter base brown colour. You can also put a tiny bit of blue in your base brown to deaden the colour, highlight it up as normal, and do a final light drybrush of bleached bone over it (assuming there is some sculpted wood grain that the drybrushing will pick up).

For fresh-cut wood, the "cut" edges of the timber should be a different colour from the rest of the timber, probably more vibrantly coloured. Over time, this disappears and old wood tends to weather into a uniform colour.

Also, the metal fixings should match: for weathered wood, the metal should be pitted, rusted, with a patina of use. This will help create the illusion!