Having approached my local MP, Iain Stewart...

I approached my local MP with this issue I have with the CMS and the response I received was: I have received a reply from the Child Maintenance Service, but I am afraid it is not the answer you had hoped for. They insist they can only go on the income declared by [my ex] to HMRC, and are unable to take into account any of the evidence we sent them, provided by you. They do say that if you have evidence of dividends or other income that has not been declared by [my ex], you can ask for a variation, but the onus is on you to provide evidence to support your application. I am sorry it was not more helpful.