Sorry to read about the problems with the Great Tit nest. My original thoughts were that they may have fledged but you are hearing them. (I have just checked and they leave at 18-20 days, so too young yet). It may be possible that that parents have been predated by something, hence not being back to the nest. Sad as it is and much as you want to help, there is nothing that you can do, as you already know it is not allowed. Have the parents been back yet?