I ported over my sketch last night to the SdFat library and hey presto the sd is now sleeping like a baby between read / write events. The current consumtion on the SD card has fallen into the circa 200uA region regardless of which card i use. That means the overall system is now running at only 13mA which is mainly the LCD screen. this will give me around 70 hours of run time from 4 AAA batteries - easily enough for the 24 hour race with practice and qualifying covered as well.
It took me a few attemts to convert the code over but its all working fine and the examples in the SdFst library were a big help
Thanks for your help with this problem and keep up the good work - I'll only be using this library from now on for future SD projects