Find the missing character?

Start in the bottom left hand corner of the diagram and, moving around the whole diagram, work in a clockwise direction, around the outer squares. Letters are written in alphabetical order, in steps of 7 letters at a time. For the single square still remaining in each group of 4, add the numerical values of the other 3 letters in the square together to give a 2 digit number, and add these separate digits together, and put the letter with this numerical value in the central square.