It says the following and I think the 18 hours is understandable when there is appropriate services running 18 hours prior to the flight but how about when there is not??
If prior to the departure date there is a cancellation, alteration, change or curtailment relating to a holiday which results in more than 18 hours change in the time of departure of any single flight, or a change in resort or type of accommodation offered, or some other change which significantly alters the holiday, the Organiser shall, if practicable, offer an alternative comparable holiday of at least similar standard or shall refund the Consumer all monies. Unless within seven days of issue of the offer of an alternative holiday, it is accepted by the Consumer in writing, the Organiser shall assume that the Consumer has declined such offer and the Consumer shall only be entitled to return of payments made.