The nicer side of Bishops Avenue. Cross the road from the rotting Tower and Jersey House feels a million miles away. The electronic gates swing open on a rebuilt L38m, seven-bedroom mansion with silk carpets and limestone floors. The suited housekeeper turns on ambient music and reveals a six-metre-high hall with shimmering bauble chandelier. A fur is tossed over a chaise longue to create a Downton Abbey ambience. Everything from the leather-clad lift to the vast master bedroom with its three dressing rooms and three bathrooms is 'reassuringly expensive' he says. The master bedroom is in fact one bedroom, three dressing rooms (two of which are fitted out in his and hers styles) and three bathrooms. The staff quarters is a self-contained flat that would delight any first time buyer. In the basement is a swimming pool, gym and spa, while there is a cinema on the first floor and there are two kitchens ? one built to catering standard that could serve a small hotel and another more for show. Books are scattered around like heavy hints: The Richest of the Rich and The Villas of the Riviera. A third of the mansions on the most expensive stretch of London's 'Billionaires Row' are standing empty, including several huge houses that have fallen into ruin after standing almost completely vacant for a quarter of a century. Am investigation has revealed there are an estimated L350m worth of vacant properties on the most prestigious stretch of The Bishops Avenue in north London, which last year was ranked as the second most expensive street in Britain. The empty buildings include a row of 10 mansions worth L73m which have stood largely unused since they were bought between 1989 and 1993, it is believed on behalf of members of the Saudi royal family. Exclusive access to now derelict properties has revealed that their condition is so poor in some cases that water streams down ballroom walls, ferns grow out of floors strewn with rubble from collapsed ceilings, and pigeon and