It's a T40. I knew I had one but couldn't remember if it was a T40, T45 or T50, but just went out with some of my bits and it is definitely T40.
Depending on what you have available, T40's seem to be most commonly available in 1/4" hex drive sets, however due to the position of the fronts and the torque required, I usually use a 1/4" hex drive T40 in 1/4" socket with a small ratchet handle.
Don't forget to unplug the pretensioner and pull the wiring through before trying to remove the seat. The plug is behind the small cover on the outside of the seat base. The cover slides up.