in debt with tax credits.. need help

gemjam_2

back in 2015 tax credits sent me a letter saying I owed 4k, from claims in 2009 to 2012, i asked for evidence as i couldn't understand how when my husband and i were on low incomes, i rang them a good 5-6 times and every time they told me they could not tell me why/how i owed this much as it was a different department which isn't open to the public?  i ended up just setting up a direct debit of £40 per month and it ends July 2022, then three weeks a go a get a letter saying they have found i owe on top of the previous amount another £423? im so overwhelmed i don't get how they are getting this information and im actually appalled how im not allowed to see how they have found these figures? anyone know who i can contact to find out more information without hitting a brick wall again?