FIX: Induction completely taken down right back to the head, all parts/seals/joints etc examined, breather valve brocken into and nothing found wrong except "gunge". New breather fitted on re-assy, and engine ran beautifully while stationary until throttle used ---then back to its old tricks with "closed loop" inactive. But when at steady idle and a light tap on the Mass Air sensor with a small screwdriver handle active C/L re-established, this reset was able to be done at will.
The Mass Air sensor changed and car has been tested over the last ten days with no further snags.
The M/A sensor used in this FIX was that as GSF supply for a VW 1800 Golf at £40, instead of BMW by chassis number for about £170 [the internal sensor piece is the same Bosch number].
My thanks to Kevin, Alan, and Wes for their kind help.