Mix all the ingredients for the marinade together in a large shallow dish.
Cut the peppers into quarters and remove the seeds. Cut the aubergine into 1cm/½in thick rounds and cut the courgettes lengthways into slices.
Add the peppers, courgettes and onions to the dish, turn until well coated in the marinade and set aside for 1 hour if you wish. At the last minute, toss the aubergines with the rest of the vegetables so that they don't absorb too much of the oil.
Lift them onto the vegetables onto the barbecue and cook over medium-hot coals for about 6-8 minutes, turning now and then and basting with any leftover marinade, until they are soft and richly coloured.
Remove to a dish and sprinkle with a little more seasoning if you wish.