Robert Thompson and Jon Venables, both 10, abducted James Bulger in 1993
Search party started at Strand shopping centre, Bootle where he disappeared
Alex Radnor-Boulton retraced steps out of centre and suggested checking CCTV
Local business AMEX found 'breakthrough' footage of boys walking off nearby
Led to Bulger's body being found on railway line and later conviction of Venables and Thompson