R (AX London) v Central London County Court  EWCA Civ 988
The county court can, on an ex parte application, make an interim displacement order under s29; it is lawful to detain a patient under s3 on the basis of it, although unless there are cogent reasons it is preferable to wait until the final order; even if the order had been declared invalid, the decision to admit the patient would still be valid.
The summary below has been supplied by Kris Gledhill, Editor of the Mental Health Law Reports. The full report can be purchased from Southside Online Publishing (if there is a "file not found" error, it means this particular report is not yet available online). More similar case summaries from the year 1999 are available here: MHLR 1999.
The lawfulness of a s3 detention imposed after an interim displacement of a nearest relative who objected - R v (1) Central London County Court (2) Managers of Gordon Hospital ex p London  MHLR 21 CA
Points Arising: A county court may make an interim order in relation to an application made under s29 MHA to displace a nearest relative; but if the patient is detained under s2, the extension of that detention by virtue of s29(4) MHA is preferable to having the replacement nearest relative consent to a s3. However, a s3 order made is valid and the hospital can rely on it by reason of s6 MHA.
Facts and Outcome: Whilst L was detained under s2 MHA, an application was made to displace his G as nearest relative under s29 of the Act on the basis that her objection to a s3 detention was unreasonable. At an ex parte hearing in the county court, of which G had not been given proper notice, she was displaced “until further order” but with a direction that the matter be reconsidered at an inter partes hearing after 7 days; there were further interim orders to continue the displacement (and a final displacement order was eventually made), and after the first interim order L was placed under s3 MHA. The county court’s decisions to make ex parte and interim displacement orders were challenged in judicial review proceedings; and the decision of the hospital managers to accept the s3 detention was challenged on the basis that the s2 detention continued (by reason of s29(4)) and so the s3 order should not be accepted until a final decision on the displacement issue. The Court of Appeal, upholding the High Court, determined that the county court’s general power to issue interim and ex parte orders was not excluded by the language of the MHA; that whilst making a s3 order should not be made on the basis of an interim displacement unless there were good reasons not to rely on the statutory extension of the s2 detention, there was no disadvantage to the patient detained under s3 following an interim displacement because if the displacement was not made final, the nearest relative would be able to discharge the patient under s23; but the hospital could rely on the application by virtue of s6 MHA, since it appeared to be valid.