Since no detail in the life of England's PRINCESS DIANA is too trivial for true royal buffs, let it be noted that Di, 8-year-old PRINCE WILLIAM and three of her security men recently trooped into the Happy Eater, a roadside restaurant along the Al motorway in Bedfordshire. Wills had a hamburger, French fries and a Coke, the security guards had teacakes and coffee, and the diet-conscious Princess stuck to mineral water. Total bill: £8 ($16). Di left a tip of £2, "The biggest I've ever had," said waitress CATHRYN MORSE. The Happy Eater's deputy manager, RICHARD POWELL, told England's Today newspaper, "The other customers were dumbstruck as the royals walked past and sat down. They stayed for 25 minutes, but nobody else left while they were here."