Preheat the oven to 200C/400F.
Cut the spine out of the chicken as per the instructions above and set that aside to use in the gravy. Cut that into smaller pieces. Press down on the chicken to open it out flat.
Mix the butter, mustard, garlic, pepper, salt, chopped herbs, lemon juice and Worcestershire sauce in a bowl until well combined.
Gently pull the skin away from the meat of the chicken with your fingers. Stuff pieces of herb butter under the chicken skin wherever possible. Most will go under the breast. Smear any leftover butter over the chicken and between the legs.
Season the chicken with salt and a light sprinkle of baking powder (this helps to crisp up the skin).
Roast uncovered for 45 minutes. If necessary and to prevent over-browning, loosely cover the chicken with a piece of foil from 30 minutes into the roasting time.
Allow the chicken to rest for a few minutes before carving.
While the chicken is roasting, make the gravy/jus.
Add the chopped chicken spine pieces to a medium pot with a splash of olive oil. Brown these pieces on all sides.
Add the chopped onion, carrot, and celery and sauté until soft, about 5 minutes.
Deglaze with ½ cup of sherry or white wine and allow this to reduce slightly. Add the stock, herb stalks and bay leaves let this bubble away for 20 minutes.
Strain the gravy removing any solids and put it back into a clean pot (or the same pot that has been wiped down). Cook until this has reduced by half.
Scrape any sticky bits off the bottom of the chicken roasting dish and add these to the gravy. Spoon in some of the melted butter too.
Cut up the chicken and serve with some of the gravy poured over or on the side or both. Serve this with vegetables of your choice or a salad and bread to mop up the sauce.