Hi Guys,
I've been perfecting this for a while and I reckon it's pretty bang on.
Like most things in life, it's all about the prep.

I'll be working off a pack of 7 chicken thighs for amounts. I use them, as the bone makes the finished product really juicy. Feel free to use, legs/breast etc

So:
remove the skin from the thighs (dead easy, just hook a thumb under the skin and rip it off)
Sharp knife: cut three parallel slits into each thigh.

Stage 1 marinade:
teaspoon of salt
1 and a half teaspoons of red chilli powder (I use red Kashmiri, - most indian shops sell it)
teaspoon of ginger paste
teaspoon of garlic paste
good glug of lemon, or lime juice

in a large tupperware, or similar, add the ingredients to the chicken and rub it all over, paying attention to the slits and making sure you coat the underside of the thighs as well.
Set aside and chill - Min of 2 hours (I leave mine overnight)

Stage 2 marinade
250g of plain greek yoghurt (if you can get hung yoghurt, even better, but a thick greek yoghurt will be fine)
teaspoon of salt
1 and a half teaspoons of red chilli powder
teaspoon of ginger paste
teaspoon of garlic paste
good glug of lemon, or lime juice
100ml of mustard oil (again, indian shops stock it, if not, mix mustard powder/paste with vegetable oil)
teaspoon of garam masala

combine all the ingredients in a bowl, and mix to form a smooth thick sauce.(at this stage, I take a bit of the sauce out and set aside for additional drizzling )

Once combined, pour over your chicken and again, get it into the crevices and crannies.
Back in the fridge - 3 hours minimum (again, I leave mine overnight)

When ready to cook, remove from fridge and let stand for at least 30 mins, so it gets up towards room temperature.

preheat oven to 200C and place chicken on a large, foil lined baking tray with a wire rack.
Cook for 30-40 mins until the tips of the chicken begin to blacken slightly.

Once cooked, remove from oven, allow to cool slightly and enjoy! (it's at this point I bring in the marinade I set aside earlier, which I've heated for drizzling onto rice etc)

Points to note:

yes, I know, this is oven cooked,not cooked in a tandoor. If any of you have access to one, I'd love to know how it turns out cooked in one of them. I also imagine it would barbecue beautifully as well.
Trust me though, even oven baked this is superb