The idea that a court’s decision to impose a hospital order itself satisfies the right in Art5(4) to “take proceedings” - at least for a period - is founded on the decision of the ECtHR in De Wilde, Oms and Versyp v Belgium (2832/66 etc), see in partic para 76.
http://hudoc.echr.coe.int/eng?i=001-57606
As I recall, that’s the basis on which the six month exclusion rule for Pt 3 patients has been justified to date.