SSCX Only!

Something like 42x18 is fine for dry courses, like it is now, but if you're riding through thick, gloomy mud then you're likely to need a smaller gear, so that wider choice on chainrings is useful.

Cheers for the tip! I think most of the local courses are in parks/fields, so grass and mud is far more likely than gravel. I'm also (unfortunately) unlikely to get the bike built up before the weather starts to turn, so it sounds like I'll need/want the flexibility!