When my bloke was paying csa money he had maybe a similar situation. He gets overtime but this is not the same amount every month. Some months he gets quite a bit and other months none. The csa said that this is why they take three months worth of wage slips as this can give them an idea of his average earnings. I think (correct me if i'm wrong) that they will only do a reassessment if you inform them in a change of your circumstances or your ex says she/he thinks your circumstances have changed and you haven't let them know. Maybe not the definate answer you were looking for though