Concentrix
Concentrix are a private company who are paid by HMRC to investigate potential Tax Credit fraud. They appear to concentrate on those single claimants who they suspect are living with another person without declaring it.
If they write to you stating they believe you are cohabiting then do not accept this, often their information is out of date or simply wrong and you should not be intimidated into withdrawing a legitimate claim.
If in doubt get help from your local CAB or specialist solicitor.