TechWiki:Cambelt Change Frequency
Lotus have amended the original schedule and now recommend changing cam belts at least every four years rather than the original six, this will often mean services can become "A+belts", "B+belts", or "C-belts" depending on age and mileage and you will need to track the age of the cambelt separately from the normal service history.
Extract from Group Lotus Aftersales bulletin ...
The synthetic cam belt used on Rover powertrain Elise models is subject to age, as well as usage degradation, and will be adversely affected by any contamination with oil or coolant. In order to bring cam-belt time servicing in line with current Rover recommendations the Service Schedule for all Rover powertrain Elise models was revised in October 2003 to stipulate a cam belt replacement time interval of 4 years (54,000miles/90,000km), with an inspection every 2 years (27,000miles/45,000km). For 340R/Sport 160/Exige models, the corresponding distance intervals are 36,000miles/60,000km, and 18,000miles/30,000km.