A prince of uncertainty: Richard Tomlinson assesses the image revealed in last night's documentary

A TELEVISION portrait lasting 21/2 hours carries its own message. The Prince of Wales wishes to be taken seriously, and Jonathan Dimbleby is pleased to oblige. Dimbleby concludes that Charles is 'at ease with his role'; but notice how the prince fidgets.

He twists the signet ring on his left little finger. He tugs at his shirt cuffs, as if to verify that his shirt is still there. He scratches his ears. Visibly, he is not a prince in repose.

Why should this be so? More than a century ago, Walter Bagehot wrote that 'all the world and all the glory of it, whatever is most attractive, whatever is most seductive, has always been offered to the Prince of Wales of the day, and always will be.'

On the evidence of the programme, Charles has his fair share of temptations to enjoy or resist. His life is sumptuously comfortable. He has a Georgian home in Gloucestershire, with an agreeable garden he enjoys showing to visitors, and the run of his family's Scottish estate. For amusement, he likes to land the aeroplanes which carry him on royal business - though after yesterday's scrape, he may foresake that pleasure, too.

And yet he frets about the royal business, in strange, convoluted sentences. Bagehot must take some of the blame for his confusion. Every heir to the throne since George V has been made to read Bagehot's English Constitution, first published in 1867. Charles's mother, for instance, was given tutorials in Bagehot by the vice-provost of Eton when she was 13, while her governess passed the time with PG Wodehouse's Uncle Fred in the Springtime.

From Bagehot, they were supposed to learn the theory behind constitutional monarchy. Instead they found a writer who doubted its practicality. 'On the continent, in first-class countries', Bagehot notes cheerfully, 'constitutional royalty has never lasted out one generation . . . we must not reckon in constitutional monarchy any more than in despotic monarchy on the permanence in the descendants of the peculiar genius which founded the race . . . there is no reason to expect an hereditary series of useful monarchs.'

Dimbleby's documentary was an ambitious attempt to confound historical expectation. Unlike his predecessors as Prince of Wales, Charles is determined to be useful; to such an extent that 2 1/2 hours can scarcely encompass his activities. 'Imprisoned by a diary from which he'd like to escape', Charles still finds time meet organic farmers in Mexico, Bedouin tribesmen in the Arabian desert, unemployed young people in Norfolk, and the Secretary of State for Employment on behalf of the Prince's Trust.

It is possible to believe - as Dimbleby clearly does - that most of Charles's work helps to make the world a better place. It is also possible to believe - as the Queen Mother clearly does - that for Charles the programme was a strategic mistake.

As heir to the throne, it is foolish of Charles to argue that politicians cannot have 'a monopoly' over issues of public concern. It is foolish to rubbish the work of the Foreign Office, to defend the sale of arms, and to complain about having to perform elementary royal duties (in this case, attending the annual royal film performance). The British like their monarchs to keep quiet and get on with it. In other words, they like the Queen.

Above all, it is foolish, even crazy, to draw attention yet again to the failure of his marriage. This was the reason why so many people watched last night, but Charles was under no obligation to answer Dimbleby's question about his adultery (in contrast to Dimbleby, whose professional reputation rested on asking it). By confronting the issue - in his usual roundabout fashion - Charles breaks the first law of political survival (which applies to would-be monarchs as much as politicians): when you are in a hole, stop digging.

Dimbleby could have asked a rather more interesting question about Charles, 'the private man, the public role'. Does he, and it really matter? Instead, from the opening sequence, Dimbleby takes for granted that this is an important subject.

Yet the debate about the monarchy rarely considers how little the Queen's constitutional functions affect the workings of the state. She could cease to read the red boxes, stop receiving new ambassadors, halt the weekly audiences with the Prime Minister, refuse to deliver the Queen's speech (as Victoria did for much of her reign), and public business would still be transacted. These rituals are symbolically important to the monarchy, not to the constitution - a fact which republicans and royalists overlook, since both have an interest in talking up the crown's significance.

Bagehot grasped this point rather better than Dimbleby has done. On last night's evidence, Charles does not appear to have grasped it at all.

'Divine Right: the Inglorious Survival of British Royalty', by Richard Tomlinson, is published by Little Brown at pounds 18.99.

Thomas Sutcliffe returns next week.