QUEEN VICTORIA: A Portrait By Giles St Aubyn (Sceptre, $175) ANYONE who believes a royal scandal is an invention of the 20th century press would do well to read this entertaining portrait of the most remarkable monarch of the last century. The parallels are extraordinary. Take 1889 for instance. By its end the Prince of Wales had survived another public humiliation by the skin of his teeth. Already known as an adulterer, it had taken the threat of exposure through judicious use of the newspapers to stop the future Edward VII from interfering publicly in the affairs of a member of the aristocracy. They needed no Edwardgate tapes; the fact that the future King of England had been toying with the man's wife was sufficient scandal to bring the royal name into further disrepute. Queen Victoria, on this occasion and others, was known to fear privately for the monarchy's future. Radicals were already questioning its value in financial terms. Overpriced and unsupervised they said. It should make way for the republic. Giles St Aubyn wrote his book before the worst of the recent scandals hit Victoria's great-great granddaughter and her off-spring. Even if he had been aware of them, such parallels would not have been part of a book which investigates the character of Britain's longest-serving monarch and traces the changes in her character and her family's progress as her reign made its way throughthe industrial revolution to the days of empire. The political complexities and the social context of these times are left to others. But there is still ample evidence here to back the theory that a monarchy that bolsters national unity by symbolising a moral creed, must be seen to be living by that creed, in public at least, or run the risk of having its role brought under scrutiny. As Mr St Aubyn explains, Victoria was the first of this modern breed of public monarch, succeeding to the throne at age 18 after the depressing reigns of two somewhat sordid uncles. The monarchy may not have survived another of their kind, a fact realised by Lord Melbourne, the prime minister of the day, who used his fresh, young queen to bolster an institution essential to the constitution, as well as the fabric, of a society whichhe wanted to see survive intact. But, as Victoria was to discover, being placed on a pedestal is one thing, staying up there is another matter entirely. First her political views came under scrutiny, then her marriage to a foreigner, a German prince, was condemned. But her worst years came with the death of Albert of Saxe-Coburg and her grief-stricken seclusion which lasted far longer than even The Timesof London was prepared to accept. Three years after his death, the Thunderer thundered: ''For the sake of the Crown as well as of the public (it had to be recognised that it was) impossible for a recluse to occupy the British Throne without a gradual weakening of that authority which thesovereign has been accustomed to exert.'' It might as well have stayed silent. It took Victoria another 10 years to return to full public duties, three years after Sir Charles Dilke, the Radical MP for Chelsea, indicted her for dereliction of duty in a speech which prompted Joseph Chamberlain topredict: ''The Republic must come.'' It didn't, of course, and it still hasn't. Chamberlain went on to become the high political priest of the British Empire; Victoria reigned until 1901 and her son, the Prince of Wales, kept any future scandals under wraps to succeed her.