I agree that the lambda sensor is a likely candidate given the symptoms but this would have flagged up on a FCR.
We do not have access to all the information the mechanic in Spain has, that means we are unable to help fully as we would like to.
Fault codes however are not an exact science tbh, It is only a small indication in the bigger picture to the mechanic. Just because a FCR flags up a fault it doesn't automatically mean a faulty sensor. The very low voltage signal back to the ECU will show only an open or closed reading which the ECU then compares with it's pre-programed map. If the ECU is unable to adjust the parameters it goes into limp mode.
The fault signal the ECU receives from a particular sensor or group of them can be as simple as a pos: + feed wire to sensor damaged, poor contacts in plug and socket, cracks to the insulation on wire, etc.
I had a fault code for TPS sensor, so bought a new one and fitted it...job done....well no. when fitting tps I moved harness to it
and only then noticed a bare wire snapped to tps. I re -soldered and heat shrunk joint.....this was now the fault...not the tps.
but the ECU didn't know this it just flagged up a sensor fault.
Reason I mentioned this is I now had 2 good tps sensors to add to box with several possibly good but discarded sensors.
After investigation of all the engine harness it was so brittle due to age and heat that the insulation snapped when bent with my fingers.
The thing that concerns me is that the collection of various different engine sensors in the box, maybe if bench tested some could be Ok. They ain't cheep......might be glad I kept them
food for thought