London, I'm at the PC now. The tablet is very awkward to post links from easily. 

I did what I usually do for any new dish and researched / read up all I could find. I found variations on the dish and extrapolated a recipe that took bits and pieces from several of them but mostly it follows the recipe from a book called Made in India Cooked in Britain.  This recipe can be found;
https://www.telegraph.co.uk/foodanddrink/recipes/10922316/Lamb-raan-recipe.html and
https://thehappyfoodie.co.uk/recipes/lamb-raan.
I also used the recipe from The Spruce Eats  https://www.thespruceeats.com/masala-raan-roast-leg-of-lamb-1957562 and
Madhur Jaffrey https://girlinterruptedeating.wordpress.com/2010/05/20/madhur-jaffreys-spiced-yogurt-leg-of-lamb/. 
I also read up on the traditional form of the dish and variations from different regions as well as the recipes for Ran in Tandoor by Ranjit Rai among others. 

Aside from basically following the Made in India recipe, I made adjustments by adding some extra ingredients found in the other recipes. I included 1 onion and a few dried figs in the blended marinade. There were a few spices not in this recipe that appeared in others so I included some Green Chillies (home grown hot little birdseyes), 1 tsp of Garam Masala and a few cloves. I nearly doubled the amount of yogurt and also added a TBSP of Malt Vinegar. I used 2 X 1" pieces of cinnamon stick instead of powder.  You just grind the spices, or use powdered, put all the marinade ingredients in a blender and blitz. Clean up and make deep square cuts in the leg of lamb and marinate it for at least 8 hours. 24 - 48 hours is recommended. 

This can be slow roasted in a home oven, cooked completely in a low Tandoor or, as I intend to do, oven first then finish in the Tandoor.  A 2.5 kg leg is recommended to be cooked over at least 3 hours so I'd say your only using 140'C - 150'C for 2 1/2 hours and finish with a higher temperature for 20 - 30 minutes to brown and crisp the outside up.  Cooked low and slow will provide more tender lamb so you could cook this over a longer time at lower temperature. 4 hours at 120'C or 6 -7 hours at 100'C.

I'll be cooking it later this evening as it's tipped to reach 40 odd 'C in the shade today.  I'm really looking forward to it.  ;D

I'll now be cooking it tomorrow after a 48 hour marinating. Aside from it being too hot today, (not expecting different tomorrow) there was nobody home to eat it. It will be cooked and I'll post pics and a review.

Chicken Tikka. you can see I turned the gas up for the Tikka after cooking the lamb slowly. 

There was quite a bit of yogurt spiced marinade left after the lamb was removed and cooked so rather than waste it I added a few large tikka breast chunks.  I guess its a type of Tandoori Chicken Tikka but not quite Makhoni Tikka. Delicious anyway.

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