First make the dipping sauce. Put all the ingredients in a saucepan and heat over a low heat without boiling for 10 minutes. Keep warm.
Meanwhile, heat 5 cm (2 inches) vegetable oil in a wok or deep, heavy-based saucepan until it reaches 180–190°C (350–375°F), or until a cube of bread browns in 30 seconds.
Quickly whisk all the batter ingredients together in a bowl. Dip the vegetables in the batter, a few at a time, add to the hot oil and deep-fry for 2–3 minutes until crisp and lightly coloured. Remove with a slotted spoon and drain on kitchen paper. Keep warm in a moderate oven while cooking the remainder.
Serve the vegetable tempura with the dipping sauce.