Tomorrow, His Royal Highness Prince Charles celebrates his fortieth birthday. Ever since he became heir to the throne at the age of three, he has been engaged in what he himself describes as "the longest apprenticeship ever", which may well continue for many years to come. Meanwhile, with very little help or guidance from the example set by his predecessors, the Constitution, or politicians of the day, it is very much a matter for him how he chooses to define the job of being Prince of Wales. just what he has made of it, and how ell he has succeeded, is the subject of tonight's documentary. Anthony Holden, Prince Charles's biographer, takes a critical but affectionate look at this complex man, known to everyone but understood by very few.