You may be interested to know that two (now ex) employees of a certain low-cost airline are taking said airline to an employment tribunal sometime this month due to the fact that they don't pay flight pay during leave. A law was introduced several years ago whereby employees who rely on special payments such as commission should be paid an average of that when they are on leave so the argument is that this should aply to crew. The argument is further strengthened by the fact that the airline pay pregnant cabin crew an average of their last three month's flight pay whilst they are sat at home.
If they are succesful then this should open the floodgates for other airlines to launch claims. A little off topic maybe but it may be worth mentioning to your union reps because I'm sure many crew are reluctant to take leave (at any airline!) due to the effect it has on their monthly wage...at the end of the day a seemingly small thing like that could make a big difference.