Thanks for the comments guys, got to say I'm really loving the sigma for this stuff, especially on low light days like that.
Just as important though the riders are starting to come asking for them, at the forums and at the tracks. Had to do a team shot for one of the teams as well, typically it was grey and drizzle first thing but they didn't come out too bad.
www.hardmuircroft.plus.com/Tain/motone