Strange creeping disappearing and hesitation?

Hello, what code was stored in the engine computer? Is this a gas engine? It sounds like a restricted exhaust if it's a gas engine. If it's a diesel engine the DPF (diesel particulate filter) might be clogged. I know they use a lot more diesel engines over the pond. We are based in the US. If the converter is clogged you can put your foot to the floor and barely get anywhere, but again depending on the fuel type the engine takes. But we need to know what codes were stored first.

All 3 of these codes can cause performance issues for the engine, even though this first U0415 code has to do with the ABS module, that module is still on the High Speed CanBus network in the vehicle, any important modules having to do with emissions or safety will be on the High Speed network, ie the ECM/PCM (engine computer), any Transmission Control module, Electronic Steering module if applicable. If there is corrupt data being sent out on the HS network, the engine computer will not function correctly and this being an automatic transmission most likely the PCM controls both engine functions and transmission functions.
We dont even have this vehicle in our data base here, but Im looking in another location for information for you. Im looking at quite a few different builds on this model. It is built in Mexico from 2002-2010, Brazil 2002-2014, Germany, etc. So you see there are many different versions of this vehicle, with who knows how many alterations.

But I am not a big fan of some mechanic telling someone "dont worry about it".
If you have access to a scan tool and can recheck for codes like the ones you are getting, and you can use a multimeter, we can try to deal with these codes in a basic manner.

The first code P0030 Oxygen sensor heater circuit is not a difficult fault to diagnose. Im going to post a basic Oxygen sensor circuit, since this is a 2004 and not a diesel engine, the oxygen sensors around that year will work the same when it comes to the heater circuit for the sensor, the heater inside the sensor does exactly what its called, it gets the sensor up to operating temperature as fast as possible to get the sensor working and keep emissions down. Since the sensor needs to be hot to operate, without the heater working the sensor will just sit at a bias voltage level and not input the correct voltage to the PCM for fuel control, this sounds complicated but its really not, on newer vehicles its much more complicated by far. But this code is the for the sensor 1, which is the oxygen sensor in the exhaust pipe that is before the Catalytic Converter, there will be 2 sensors, one before the Cat and one after.
You'll have to excuse my crude drawings below, but the diagrams 1 and 2 are for testing the front oxygen sensor heater circuit. the first test is a resistance test of the heater element inside the sensor with a multimeter set on the Ohms setting (diagram 3).

Now the U0415 code is other modules on the network telling on the ABS module that it is sending out corrupt data packets, it might be the cause of the other U0401 code. A way to check this is with the key Off, unplug the ABS module, turn the key On, engine Off, clear the codes out and see if the U0401 comes back right away. You may need to cycle the key On twice, since it might be a 2 trip code setting criteria. If the 401 code does not come back you know the ABS module is causing the PCM trouble communicating. But you will need to replace the ABS module if its the cause, because it might be required to be on the network, since Can bus networks require 2 terminating resistors which are built into 2 of the modules on the that network and the ABS might be one of them. See what codes set with the ABS unplugged but dont drive it with the ABS unplugged.

I do see we have info on a 2011 Ford Fiesta 1.6L, diagrams 4-7 are for the 2011 US model so I doubt they are the same, but you can look at the front Oxygen sensor wiring and see if it is the same colors at least.
But this is as far as I can go, hopefully some of this info could maybe help your mechanic figure out whats going on..

https://www.2carpros.com/articles/how-an-oxygen-sensor-works

https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor

Okay, just remember even though the ABS module may appear to not be working it might still be sending out corrupt data on the network, especially when the vehicle is moving, so unplugging it and seeing if its the cause the 2nd "U" code, the Oxygen sensor heater will definitely cause running issues since its the first sensor before the converter. If the sensor has been like that a long time, most likely the catalytic converter may have been damaged and is now restricting the exhaust. A visual inspection of the front of it inside the pipe will tell you a lot. I have had vehicles with a bad front oxygen sensor ruin the Cat on that side of the engine to where I had to replace the sensor and the Cat to resolve the running issues for that bank of the engine.