IR-35 is being replaced, or rather enhanced, by the Intermediaries legistlation in the UK
Since last year if you provide more than one person to ANYONE you have to regularly file their details (and payments) including their Tax number with HMRC. Also details of the end user. HMRC will be able to trawl & cross reference all sorts of info to see who is only working fo one company and how little NI & tax they are paying