Put the flours and ground spices in a bowl and slowly pour in 250ml water. Stir with a wooden spoon or a whisk to make a bright yellow batter. Add the salt, then put to one side.
Cut the cucumber into long ribbons with a speed peeler and keep to one side. To make the dressing, mix the yoghurt with the coriander and the juice of 1 of the lemons. Add a splash of olive oil, season, and put to one side.
Cut the leek, carrots, spring onions and red onions into thin strips - try to get them the size of long matchsticks. Mix with the chillies and the ginger.
Pour the oil into a large non-stick frying pan and heat until very hot. Mix the vegetables, a little at a time, with some of the batter. Add a pinch of mustard seeds and scoop out a tablespoon of the coated vegetables. Press flat with the back of a fork and carefully drop into the hot oil. Repeat with the remaining vegetables - you may have to fry the bhajis in batches. Move them around a little in the hot oil (with a pair of tongs) to make sure they aren't sticking together. Fry for a minute or so on each side until crispy, then drain on kitchen paper. Place some rocket and cucumber ribbons on individual plates, drizzle with the yoghurt dressing and top with 2 or 3 bhajis. Serve with the lemon wedges.