Ian,
I had the exact same problem at the Donnington with the 5500rpm misfire.
My symptoms were that when you started the car it was ok until warm - sorry don’t know the engine temp - but then once warmed and you exceeded 5500rpm in THIRD gear it would then enter safety mode (I’ve now been told) and limit the revs to 5500 rpm in ALL gears. The big thing for me is that until it was warm I wasn’t thrashing it so the revs climbed and fell above and below 5500rpm - and this seemed to be a factor as explained below.
I disconnected, and cleaned all the connections on the speed sensor and went out again and it seemed to hold out, but as the car was warm I accelerated hard and kept the revs high and it seemed to hold out for the rest of the day.
On the way home, once left to cool, the misfire returned. My wife, who was behind me in the company hack followed me into the services, I got the tools out and we repeated what I had done at the track, but I can only think that due to the lower engine speeds on the motorway the misfire returned.
So the car went into Nick Whales where Ian and the team had a look at it.
They suspected the wheel speed sensor, as this was the fault in the ECU - and they “had seen a few” apparantley !!!
This was changed and seemed to cure it when you were giving it full beans, but whilst driving home on the motorway under light throttle it misfired again at 5500 rpm so back we went.
Next was a full wiring check from the sensor back to the ECU - no problems
Tried another speed sensor - still the same.
So - sensor ok, wiring ok - left the ECU, which we changed with a spare that Ian had for testing purposes.
ECU fitted - runs like a dream - no misfire - engine pulling better - and strangely running 2-3 degrees cooler than the old ECU - both on the STACK and the SPA temp gauges.
I am currently waiting for a decision from Lotus as I feel somewhat disappointed that after 8k miles, on a car bought new, with a full dealer service history that they expect me to pay for a fault that is well documented and was the subject of a factory recall.
My symptoms appear similar to Steve’s in that it started to run roughly and the idle speed was erratic before it all seemed to go pear shaped
Anyway car is running ok now but I am waiting for an answer from Lotus on the ECU - one thing is for sure I won’t be paying for another 190 ECU from Lotus - either they pay or its a trip to DVA for an emerald to be fitted.
Best of luck of tracking down the “gremlin”.