Chris,
Coincidentally I was fixing the steam trigger in mine yesterday. The screw is a T20 tamperproof TORX- bear in mind its the type with the pin in the base.
I got my TORX set from Radio Spares (rswww.com cat no. 361-1776) a while ago - they also useful for getting into mobiles, chargers and lots of other things that manufacturers (understandably) don't want you to tamper with.
Good Luck.
Stu
February 2008