Stab the lamb all over to the bone with a sharp knife to enable the marinade to penetrate the meat.
In a bowl large enough to hold the meat, mix all the remaining ingredients - the marinade and the Spices - into a paste.
Put the meat into the bowl of paste, and poke the paste into all the gashes, ensuring the meat is well coated.
Leave in the fridge to marinate for a minimum of 6 hours, maximum of 30. The longer you leave it, the better the paste will seep in and adhere to the meat during cooking.
Preheat the oven to 350°F/180°/Gas 4 maximum, and slow-roast the lamb uncovered, but feel free to cover it if it browns too quickly for about 3 hours or so.... When really tender, the flesh should literally fall off the bone (so don't undercook it - you're not aiming for 43°C rare roast lamb here). Prior to serving, let it rest for 30 minutes or so in a low oven.

An optional garnish is to press roasted or fried whole almonds into the gashes then sprinkle with fresh coriander.

Prick the aubergines, put in a roasting tin and dress with ghee. Roast at 180°C.
Slice the onions into reasonable pieces and throw into the roasting tin. Remove when the onions are nicely browned and the aubergines cooked.
Meanwhile, grind the remaining onion, garlic and ginger to a paste. Fry in more ghee until cooked, add the dry spices and cook until oil separates, add the tandoori paste and cook until oil separates. Add the tomato purée and cook until oil separates. Add the cashews.
Add the yoghurt a couple of tablespoons at a time, stirring, cooking and reducing each quantity until you're happy with the flavour. Season. Peel the aubergines and dice the flesh into chunks. Add to the curry. Cook until blended in. Add the tomato quarters and cook until beginning to break apart. I'm starting to think it's better without those great hefting chunks of tomato in there - so perhaps chop them smaller than quarters?
Serve with the roasted onions on top.

Purée the garlic, ginger and mint with the lemon juice.
Dry roast the seeds and grind them.
Add the spices to the paste.
Add oil and a little vinegar to blend.
Fry the paste until it colours and the oil begins to separate.

Blend the non-yoghurt ingredients to taste, add a little yoghurt or water if necessary.
Stir into the yoghurt.

Whisk the ingredients together with a hand blender.
My version was a pretty close match for the Spicy Cottage version, but slightly too sweet.
Probably I used too much mint (and too little coriander), too sweet a yoghurt (greek style), perhaps a little too much tamarind (which is also quite sweet) and no green chilli.

Whisk up fresh mint, coriander and deseeded green chillies with lemon juice, salt and a little demarara sugar using a hand blender
Stir into yoghurt with amchoor and chilli powder.
This seemed to be about the right collection of ingredients (maybe a touch of mint sauce, but not too much). Getting the balance right is the tricky bit though and the original is quite sour and watery.
Mind you, any variation is pretty nice!

• ½ cup yoghurt
• 1 Tablespoon whole (unchopped) coriander leaves, or slightly less
• 1 teaspoon whole (unchopped) mint leaves
• ½ teaspoon amchoor
• 1 chilli
• ¼ teaspoon salt

This is very close, very close indeed. The authentic sauce manages to be both slightly sourer and slightly sweeter than my attempt.
I whisked the whole lot up together to purée it, but this leaves a very runny sauce, so I think it would be better to whizz up the ingredients first and then stir them into the yoghurt as above.
Again, maybe a touch of lemon juice and sugar?