Put 300ml water in a saucepan and bring to the boil. Whisk in the tamarind, sugar, cayenne and cumin and cook for 8-10 minutes until slightly thickened and syrupy. Allow to cool slightly, then transfer to a serving bowl or jar.
Bring a large pan of water to the boil and add the potatoes. Bring back to the boil and simmer for 12-14 minutes until just tender. Drain well and shake dry in the strainer. Once cooled slightly, crush lightly with the back of a knife.
Meanwhile, add the cumin and fennel seeds to a non-stick frying pan. Toast for 1 minute until fragrant, then season well. Stir in the remaining spices.
Toss the potatoes and chickpeas with the oil and spice mix. Heat the ghee in the same pan used for the spices and cook the mix for 6-8 minutes, stirring frequently, until both potatoes and chickpeas are tender and crispy. You might have to do this in two batches, depending on the size of your pan.
Serve the potatoes and chickpeas sprinkled with the chilli and coriander. To garnish, spoon over the chutney and drizzle over the yogurt.