The best I can do at the mo is about 500uA in sleep mode (the arduino bit accounts for 22uA) so I'm wondering if I need to do anything more than close the SD file to get the SD card to sleep?
Closing the file does nothing.
The standard SD.h library is based on an old version of SdFat that does not put the SD in sleep mode. New versions of SdFat enable sleep mode after every transfer and the card will go into low power mode when it becomes idle.
Almost all modern cards go into a very low power mode since this is important for battery powered devices. The 30 uA achieved by user arduino_ above is typical.
The method of level shifting or not applying proper voltage to the SD can cause higher power drain.