ONE yardstick of the London housing market in the booming Eighties was the case of the broom cupboard in Knightsbridge that fetched pounds 36,000, as first-time buyers scrambled to get on to the housing ladder. A price-for-size equivalent today, as car theft continues to spread, is the lock-up garage. One such shoe-box in Maida Avenue, west London, with an unexpired lease of 115 years, went recently for pounds 90,000. The adjoining house was sold separately for about pounds 900,000. Noel De Keyzer of Savills, which handled the sale, told the Hampstead and Highgate Express: 'It reflects the demand from owners of expensive cars to procure secure off-street parking at almost any price.' John Wooden, of the British Association of Insurers, said lock-ups meant discounts on car insurance. 'The logic is simple. Cars left on the street are much more vulnerable to theft. Those in lock-ups are much safer. Hence the discounts.'