Child maintenance using my company car as income

Hi I'm new here so apologies if this has been asked before.

I have just received this year's calculation, stating I earn £11395 more than I actually do earn, which, came as a bit of a surprise especially when it increased my payment amount despite no change in my wage for the past 2 years, so after looking into where this figure came from it seems that my company car BIK figure is being used by the CMS as "Income" even though I receive none of it as actual income and actually pay a lot more tax so in affect receive less money every month.

Has anyone actually spoken to them about the figures they use? As using a non monetary benefit to get to a monetary calculation seems a bit odd. I don't begrudge paying for my child but how does me getting a car from work make a difference? If I gave it back my income wouldn't change to suit so I'm at a bit of a loss how a company car which I need to do my job (Electrical traffic signals engineer) affects my maintenance payments.

Any advice would be appreciated.