Thx for the comments guys. I spoke to Comma Technical Support (Vince) and was told that both products would be alright to use in a Sigma engine in a Caterham. On looking at the data sheets for both coolants, the Super Longlife Red does not have silicates and other additives and the G30 does. The Super Longlife Red is recommended for Ford, the G30 does not mention Ford in the manufacturers listed. These seems to indicate the Super Longlife Red to me - pity the handbook isn't more helpful.