put these in a food processor:
3 shallots
4 cloves of garlic
6 scallions
1 tablespoon black peppercorns
1 tablespoon sesame oil
1/2 tablespoon red chili flakes
1/2 tablespoon turmeric
1/2 tablespoon paprika
1 teaspoon cumin
one handful of parsley and cilantro
3 tablespoons salt
4 tablespoons lime juice

process everything until it is a thick paste. You may have to add a little extra oil but use canola or even olive oil because sesame oil is super strong.

i am not sure the key to this is the marinade, and may lie more in the cooking and serving sauce, but I have very good luck with good amounts of salt, cayenne pepper, some red chinese chilis, and something tart like lime juice.

Looks like Nando's uses a peri-peri aka piri-piri sauce. You can find multiple sites with recipes, and depending on the type and number of peppers you use can make it hotter or milder. The simplest recipes use a combo of white wine vinegar (1 part), EVOO (3 parts), garlic and chilies. Some recipes also incorporate lemon juice, sweeter peppers, parsley, oregano, ginger, etc...