Concentrix Tax Credit Robbery

Hi PW. Please publish to make others aware of Concentrix!
My ex partner used my address simply as C/O. He was my ex for a reason namely because he is an heroin addict. We had no relationship for years but I supported him when he came out of prison after serving 7 years. 
Nothing connected us apart from him using my account for his wages. He worked so I was pleased this was his first job ever but saw nothing of his wage because he was in active addiction and still is! 
I received a letter off a company called Concentrix who work on behalf of tax credits. They have taken it upon themselves to stitch me up saying I should not have made a single claim and that me and my heroin addict ex should have made a joint claim!!!!! Are they for f******* real these robbers!!!!!
They claimed to have wrote to me in July but I received no letter off them and hey presto I am left with no tax credit money to support my daughter. 
Can this company assume someone guilty without even looking in to a persons circumstances? Cheeky hard faced bastards! Perhaps they are not trained on people supporting ex offenders and junkies? How does one have a ''relationship'' with someone who is a heroin addict? Thanks to Concentrix I have no money! 
From a furious hard working single Mum!