BLINGO UPDATE: looks like it is indeed the ECU, so a replacement is being sourced, as well as everything downstream that needs doing at the same time. Is this sensible? Arguably not. Am I going to spend the money anyway? Yes, yes I am, because I really like this car.
DYANE NEWS: while changing the 20W50 running-in* oil for Total Racing 10W30, we discovered some evidence of an exhaust blow on the offside. This was due to a loose exhaust manifold nut, so as a precaution they were all changed for the flanged BMW items we use on the racer, then torqued down properly. An iffy inlet manifold stud was sorted too (helicoil) while we were in there. The car was then lent to Laurence for the week for a proper tune-up. This thing FLIES!!
ACHTUNG!: the C180K continues to perform to specification.