Just a note for the record, in case anybody has a similar problem. My D2 was doing this intermittent flashing M + S lights, which would usually go away with a switch-off/switch-on of the ignition. Talking to the previous owner, this issue had been years in existence. Last week it finally went to limp-home permanently. We had checked all of the usual suspects as described above, including changing the inhibitor switch (XYZ) with a known-to-work used replacement. No dice. Yesterday had the tranny ICU replaced (under the passenger seat), and all good. So it was the ICU itself.