Ask your chicken monger to butterfly your chicken (unless you know how to do it yourself). Then take home.
Pat the chicken dry on both sides (esp the skin) with a dry cloth or kitchen towel. Cover both sides with a generous amount of salt. Put salted chicken in uncovered bowl then place in fridge for 2.5+ hours (I have marinated for up to five - keen to go longer if I were more organised).
Get kettle bbq started. Once flames have disappeared, put lid on bbq then wait for 20 minutes to ensure the coals aren't too hot.
Wash all of the salt off the chicken then dry off with clean cloth or kitchen towel. Make sure completely dry.
Place bone side down (skin up) on cooking rack of kettle bbq. Cover then leave for 90 minutes or so.
I saly the chicken and leave it in the fridge uncovered for two days, same system as Zuni Cafe roats chicken, and it's delicious and so juicy and tender. Excellent way to raost chicken, except I do it in the oven on 220 degrees C.