Loss of power at around 3000rpm or on hills (only when engine warm)

[CuriousGeorge]

I have a Fabia 1.2 HTP Petrol (2008).  About six months ago, the engine started pinking slightly when in 5th gear on flat roads at around 2500-3000rpm.  Barely noticeable.  But it steadily got worse over the next couple of months, now mainly losing acceleration when going up hills.  But it only ever happens when the engine is at running temperature.  I can usually get the power back if I drop down a gear.  I'm getting codes 16556 (system too rich) and 16518 (o2 sensor circuit, no activity detected).  Both codes together suggest a problem with the Lambda Sensor: either dirty or faulty.  But I'm wondering if that would that explain why the loss of power only happens when engine is warm?  Has anyone else had this problem and solved it?  I was thinking it might be possible to programme the engine to be in open-loop mode all the time - to bypass the Lambda Sensor altogether?  Or would that cause me problems with fuel economy or cold starts?  Any thoughts gratefully received.  Thanks!