P0011 - A quick google shows that this code is related to the cam position sensor. Either the cam position sensor itself is faulty, of the cams are not in the correct position and the solenoid/actuators are not operating. It could also be that the timing chain has skipped a tooth but probably unlikely.
https://www.700r4transmissionhq.com/p0011-nissan-micra/ - Gives all the issues you are getting - poor idle, poor MPG, loss of power. Nobody over the internet can really tell you exactly what the problem is without checking it..
Some further reading: https://www.micra.org.uk/threads/read-fault-codes-micra-dig-s.67184/
A whistle - could this be the supercharger? Is the engine light on still at all or in limp mode (IE loss of power and wont rev beyond 3000-ish rpm?), how far have you driven since clearing the fault, or did the code self clear? A whistle whilst the supercharger is generating boot pressure could possibly indicate an air leak? Problem with modern cars is whilst the basic principles are 'rotation, air, fuel and spark' - getting the stars aligned and all of the sensors showing the thumbs up can cause diagnostics to be tricky without 'the computer' to interrogate the ECU's. Reading a fault code gives you some direction but is more often part of the symptom rather than the fault!
