Caterham Parts have TR6AP-13 for "SPARK PLUG DURATEC STANDARD MODELS". (But different for a 485.) John Vine in 2014 wrote:
"My 2008 R400D came with Ford's 5M5G-12405-AA TR6AP-13. These are made, AFAIK, by NGK. Gap is 1.3mm. I think some owners run the (slightly harder) NGK BR7EFS instead." Further discussion in 2020. Jonathan