Mix together the salt and buttermilk.
Place the chicken into a large ziplock bag and pour the buttermilk over the top.
Seal the bag, removing as much air as you can and use your hands to move the buttermilk around the chicken.
Refrigerate for at least 4 hours, preferably overnight, and up to 48 hours.
When ready to cook, take the chicken out of the fridge and line a tray with foil or parchment. Over the sink, remove the chicken from the bag and discard the buttermilk. Tilt the chicken so that any buttermilk inside runs out. Place on the lined tray.
Preheat the oven to 390F / 200c.
Mix together the ingredients for the garlic and herb butter.
Pat the skin of the chicken dry with some kitchen paper.
Use a spoon to work away the skin from the breast. Gently pull away the skin at the end of the chicken, opposite end to the legs, and use a spoon to work a pocket over the top of the chicken. Go gently so that the skin doesn't break.
Place the garlic herb butter under the skin, and use your hands to work it all along the top.
Stuff the cavity (between the legs) with the herbs and garlic. Pierce the lemon with a fork several times and place that in. Rub in ½ tablespoon of salt into the skin.
Place in the oven and roast for 1 hour to 1hour 15minutes, uncovered.
If your oven runs hotter at the back, rotate your baking sheet every 20 minutes or so, so that the chicken browns easily.
After about an hour start checking for doneness. An instant read thermometer should register at 160F when inserted in the thigh (the chicken will continue to cook up to 165F once removed from the oven).
When ready, remove the chicken and place on a chopping board and loosely cover with foil. Let it rest for 15 to 30 minutes before carving and serving.