Heat the oven to 190°C (Gas Mark 5 / 375°F).
In a large bowl, toss the potato chunks in the oil and 3tbsp of curry sauce (not all of it!). Season with a little salt and pepper, and transfer to a large baking tray. If possible, spread the potatoes out into a single layer. Roast for around 20 minutes, until about half cooked.
Transfer the half-cooked potatoes back to the large bowl, and add the onion, leek, tomatoes, and another few tbsp of curry sauce. Mix well. Return to the baking tray, and spread everything out into an even layer (don’t worry if it’s heaped up a bit). Roast for a further 25 minutes, until all of the vegetables are soft.
When the veg are ready, return them to the bowl one last time. Add the remaining tikka masala sauce (add as much or as little as you like to get the consistency you want), along with plenty of fresh coriander. Mix well, and serve warm or cold.