Tomatoes are thirsty plants, and a week above thirty degrees will show you exactly how thirsty. The good news is that they are also forgiving, as long as you water deeply, water early and keep the soil covered.
Water in the morning, before the sun reaches the bed. Aim at the base of the plant, never the leaves, and give each plant a slow soak rather than a quick splash: a full watering can per plant every second day does more good than a cupful every evening.
Mulch is the second half of the job. A layer of straw or grass clippings five centimetres deep keeps the soil cool and cuts evaporation by more than half. Pots dry out fastest, so move them into afternoon shade if you can, and stand them in a saucer during the hottest days.
If leaves curl at midday but recover by evening, the plant is coping. If they are still limp the next morning, water at once and add shade cloth for a few days.