Place the chicken, breast-side down on a large chopping board (place 2-3 sheets of damp kitchen roll under the board to stop it slipping). Using poultry shears or strong kitchen scissors, cut along one side of the back bone. Do the same on the other side, removing and discarding the backbone.

Turn the chicken over, open it out like a book and firmly press down on the breast bone to flatten it. Slash the thighs with a sharp knife. Place in a snug-fitting roasting tin.

Blitz the coriander, garlic, lemon juice, oil and seasoning mix in blender until smooth. Pour the marinade over the chicken, rubbing it well into the slashes. Cover and leave to marinate in a cool place for 1 hour (or 2-3 hours in the fridge).

Preheat the oven to 180°C (160°C for fan assisted ovens), gas mark 4. Preheat the barbeque to a medium- high heat.

Cover the chicken with foil and roast in the oven for 1 hour – 1 hour 15 minutes, until thermometer reads at least 75°C on instant-read meat thermometer inserted so tip is in thickest part of inside thigh and does not touch bone. Juices should run clear and legs should move freely when wiggled.

Rub the sweet potatoes with the extra oil and wrap each one in foil. Place on the hot barbecue and cook for 40 minutes- 1 hour until they feel soft when squeezed. Turn occasionally and open the foil to check they are not burning after 30 minutes. Move them to the cooler edges of the barbecue, if necessary.

Carefully lift the chicken out of the roasting tin and place skin-side down on the hot barbecue rack (see Tips). Cook for 5-8 minutes (depending on the heat of the barbecue) until the skin is lightly charred and crispy in places, brushing with the juices from the roasting tin once or twice.

Transfer the chicken to a platter and garnish with the extra coriander. Serve with the barbecued sweet potatoes, split open and topped with the soured cream and chives.