Had a swing at the achari recipe from the Ashoka Simply Jiggy book. The recipe doesn't specify which "mixed pickle" to use, so I went with Pataks. 24 h marinade.

Was OK, but nothing spectacular. Not really the taste I'm looking for in achari tikka. To be fair the recipe in the book is actually for king prawns.
Plan more experiments. There is a restaurant I know that does a mind-blowing achari. Really need to see how it's done, and the pickle used. Gather it's a proper little short-cut. "Un-marinated" chicken is first cooked in the tandoor. It's taken out before it cooks/browns fully, and then pan-fried with oil, garlic paste, sugar, possibly something else, and a pickle. Heaven!
Rob