Whisk the olive oil, vinegar, mustard, mayonnaise and sugar together with a good pinch of salt. Set aside about 3 tablespoons of it.
Cook the pasta in well-salted water for **1–2 minutes longer than the packet says** — tender, just short of soft.
Drain but **don’t rinse**. Tip into a large bowl and immediately toss with about half the dressing while the pasta is still hot. It will absorb most of it.
Spread the dressed pasta on a tray to cool quickly to room temperature, about 20 minutes.
Return to the bowl and add the remaining dressing (not the reserved 3 tablespoons), then the tomatoes, capsicum, cucumber, red onion, olives and most of the herbs. Toss gently.
Fold through the feta last so it stays in pieces rather than breaking up.
Season generously and chill for 30 minutes.
**Just before serving, stir through the reserved dressing** and scatter over the rest of the herbs.