Had this problem on Saturday at the csp cx in clitheroe, got to the top of the hill on lap one, remounted and it snapped, I was fuming!
Im on a run of bad luck at the minute with another one a few weeks ago at Bolton!
Tried spraying silicone dashboard shine all over the drivetrain which worked on wetter courses. However the leaves and grass still get stuck in there, so having a spare bikes the way to go!