CH225530 | How to do a compliance check: information powers: third party notice: groups: notice to external third party to check one or more known subsidiaries
From HM Revenue & Customs · Compliance Handbook
Where a notice is given to an external third party to check the tax position of one or more known subsidiaries, but not the parent, the special rules set out at CH225510 do not apply. The usual rules about giving third party notices apply see CH23620. In summary
if you decide to obtain the approval of the taxpayer(s) rather than the tribunal, you must get the approval of each subsidiary
you must send a summary of reasons to each subsidiary
your notice must name each subsidiary that you are checking
a copy of the notice must be sent to each subsidiary.