 Complete the following phrase:  Walker had some international racing experience early in his career in the Australian rounds of the Tasman series in 196465 on challenging tracks like Longford and Sandown at a time when most Australian National 2.5 drives were near world class and he also finished 5th in the ex Follmer Lotus 70 in the Nov 1970 Australian GP at Warwick Farm. While few would have been surprised that Walker failed to match the fastest Australian F5000 driver Frank Matich, Neil Allen and Kevin Bartlett, it was a pointer to Walkers later big car problems that at Warwick Farm he was slower than Australias leading 2-litre single seater exponents, Kevin Bartlett, Max Stewart and Leo Geoghegan . During the 1960s Walkers racing career faltered he was the 1969 British Formula Ford Champion and finished third in the 1969 European Formula Ford Championship, however finally broke through racing a Lotus in Formula Three during 1971. He won 25 out of 32 races that year, including the Formula Three support races at the Monaco Grand Prix and the British Grand Prix at Silverstone. By the end of the year he had won both the Shell and Forward Trust UK Formula Three titles. Coming to the attention of Lotus founder Colin Chapman, Walker was handed his Formula One debut at the 