The breather was short lived I'm afraid as relative discharged himself quite quickly - and no, the family wasn't informed that he'd left hospital. Grrrr! 
Anyway, a new caseworker (an AMHP) has been assigned. She took the trouble to read the case notes and informed us that entries ceased on February 28th (NONE of my recently sent paperwork was on the file - even though I have a letter of acknowledgement for it), she interviewed people who knew relative BEFORE his illness worsened, and - steps back in amazement - phoned the family to ask our opinion! (Can you hear the Hallelujah chorus in the background?) As a result of her diligence she has, in the space of a week, convened a case conference, got her paperwork lined up, found him a hospital bed and has been granted a Section 3 order!! There are indications that she also sought a second opinion from a different psychiatrist. Honestly I am so thrilled. We're not out of the woods yet because when the team went to serve the order this morning he'd already left the house; he's due to visit this area for a few days from today and we could get into difficulties with him, but at least we now know that an end is in sight.
Thanks to everyone for giving me a safe space to sound off about this; I've really appreciated your encouragement and support. 