The position with your Income support is that an overpayment of benefit is recoverable if you failed to disclose a material fact or you misrepresented a material fact (i.e. made an incorrect statement).
You may also be in receipt of Housing and Council Tax benefit in which case the LA must not recover an overpayment if the overpayment was made by an Official error.
In my experience one of the most likely reasons will be that someone has said your ex-boyfriend lives with you and the DWP think you therefore did not disclose a material fact.
If this is untrue and you have promptly informed the DWP of any change of circumstance you should have nothing to fear, you can appeal against any decision they make within one month of the date of their decision letter.
You are obviously concerned about the DWP letter, they do write in an accusatory manner, this is normal, and there are many people on AB with knowledge of benefits that will be pleased to help.