I've read the above here before - and indeed even been told that once back when in the FIRST lounge at T1 (yes, I put it in all caps, because it was that long ago).

However, every time I have connected through T5 (longhaul to shorthaul) I have been able to have a treatment (as long as they were not fully booked). I simply show my arriving longhaul boarding card (the same as what I show for Concorde Room access).

Given that Elemis even describes one of the chair settings as ideal for "reviving after a long flight" (or however they put it), it would be rather strange if they then went on to say that you can only have it before your flight (which you've already taken!)

My instinct is that this has changed either with the Molton Brown to Elemis switch, or with the T4/T1 to T5 switch, as you used to only be able to get a treatment on a long layover by clearing immigration and going to the Arrivals lounge.