Cat has a test kit #223-9116 ELC Dilution Test Kit. I'd recommend you test your system.....seems I recall five years is about the expected life but I suppose as long as things test Ok it's still good.
OM, many thanks for the prompt response. It's starting to get cold in the UK so I thought it would be good to check whether Its time to change the ELC before the traditional deep freeze start in the new year. No doubt the KOO would regard a UK winter as just a mild frost in the Kingdom!