1. Peel and grate 3 garlic cloves into a large dish. Squeeze in the juice of 1½ lemons and add a generous pinch of salt and pepper. Add the chicken half to the dish. Rub it all over with the lemon and garlic mix, then pop the dish to one side to marinate for 1 hr.
2. While the chicken is marinating, peel the potatoes. Slice them into chips about 1cm-thick and slide them into a large pan. Cover the chips with cold water and pop the pan on the hob. Bring the pan to the boil, then reduce to a gentle simmer. Cook the chips for 5-6 mins, then drain well.
3. Warm your oven to 200°C/Fan 180°C/Gas 6. When the chicken has marinated for 1 hr, drain away any excess liquid. Transfer the chicken to a roasting tin. Pour over 1 tbsp oil and add a good pinch of salt and pepper. Rub the chicken with the oil and slide the tin into the oven. Roast the chicken for 45 mins.
4. As soon as the chicken goes into the oven, pour 2 tbsp oil into a separate, large roasting tin and add a pinch of salt and pepper. Slide the tin onto a shelf below the chicken and warm up for 10 mins. When the tin is hot, remove it from the oven. Carefully add the drained chips to the tin and use a spatula to turn them in the oil. Slide the chips into the oven and roast for 40 mins, turning halfway, till golden and crisp.
5. While the chicken is roasting, make your piri piri sauce. Trim the tops off 4 chillies and roughly chop them. Peel and roughly chop the remaining 4 garlic cloves. Pick the thyme leaves. Add the chillies, garlic and thyme to a small food processor. Squeeze in the juice from 1½ lemons and pour in 50ml red wine vinegar. Spoon in 1 tbsp smoked paprika, 1 tsp dried oregano and 1 tbsp sugar. Season with a pinch of salt and pepper and turn on the processor. Whizz the sauce until well combined and smooth, then pour into a small bowl. No processor? Finely chop the chillies and thyme, grate or crush the garlic, and mix with the remaining ingredients to make a chunkier sauce.
6. When the chicken has roasted for 45 mins, remove the tin from the oven. Spoon over most of the piri piri sauce (save a few spoonfuls for serving) and use a brush to evenly distribute it. Slide the chicken back into the oven and roast for a further 10 mins. The chicken should be cooked through – if you slice into the thigh, the juices should run clear. If not, cook for a little longer and check again.
7. When the chicken is cooked, transfer it to a platter. Serve with the chips and remaining piri piri sauce on the side, for dipping, and your favourite sides and salads.