Crush the chillies and garlic together with a pinch of salt with a pestle and mortar (or in a blender) to make a fine masala paste.Put the batter ingredients and the masala paste in a medium bowl and gently mix, working the spices through it, to form a relatively runny batter. Cut the potatoes in 5mm-thick slices. Mix the coriander seeds and pepper in a small bowl.Heat the frying oil – about 15cm deep – in a large pan over a high heat (or in a deep fat fryer, if you have one) and when you think it is hot enough, test the temperature by sprinkling a few drops of batter in the oil. When it is up to temperature, the drops will quickly spring back up to float on the surface. Reduce the heat to medium.Put four potato slices in the batter and work them around to make sure they are fully coated. Lift one slice out of the batter by its edge, hold over the bowl for a moment to allow any excess batter to drip off, then turn to hold horizontally. Sprinkle the top surface with seed-pepper mix and carefully place in the oil sprinkled-side uppermost. Repeat with the other three battered slices. Some batter droplets may run off the potato slices – use a tea strainer or small sieve to remove them so that they do not burn. After three minutes turn the slices over with a slotted spoon or strainer to cook the other side. Cook for a further 4-5 minutes until the batter is a crispy golden brown and the seeds are dark brown. Remove from the oil and leave to rest on absorbent kitchen towels while you batter and fry the remaining potato slices in batches of four at a time.