working tax credits

I have just received a bill from tax credits stating i owe them £1,186.89 from 2007. When i called to query what this was in regards to i was also told i owe a further £1,300 from 2009/10 this is in addition to £2000 we are already paying off in instalments from another year. My husband has also had his tax increased on his wages because of them giving him the wrong tax code in 2009/10. At this time we are desperately trying to clear our debts off but i feel like i've been shot in the head with these new debts. I had had no previous correspondence regarding these debts, though the tax office are claiming they sent them. Is there anything we can do to fight this, as this is now 5 yrs down the line and we are barely managing on what we are earning as it is. please help we are desperate.