Like I said, every 25,000 miles. If you can assume it was done at 25,000, then at 40,000 you don't need it doing. More likely next year.
It's 25,000 or every 2 years (although it's probably not really necessary) mine was last done at 25,000 but that was about 4 years ago and I only do 3,500 miles a year so it's unlikely i'll get to 50,000 while I still have the car so I figured I may as well get it done this year.