Low Oil Pressure warning, Stop Start fault

My first post here so hopefully in the right place as covered by the 'bleeping' category, if not please move it.
C3 2006 petrol 1.4 16v Stop Start Sensodrive. Note battery was original and would start the car ok despite accepting a charge next day it didn't seem to need.

The car was serviced by a local Citroen garage here in France who replaced the timing belt and did a full service. It drove for 100km then on a short trip I got Low Oil Pressure warning, they checked it and all ok (cleared old faults) and the engine sounds fine, no rattling so I believe oil pressure is ok.
Went home and ok for a 10km round trip then started again, this time with an additional Stop Start fault.

I changed the battery, Yuasa 65ah 640a, and tried again but still get the same messages, now even more insistent on the oil pressure.
So I will change the oil sensor today as it may be failing anyway, cheap so no big deal.

Next thing is to get the garage to reset the computer to factory settings, if they can, so it wakes up and reinitialises. Hopefully this will reset all sensors etc and recognise the new battery ratings as I heard this can be a problem. Anyone had this and how well did it work ?
Regards John

HI

You can't reset the ECU to factory settings, there is no such thing.

Just fix the fault and the error code will be gone.

Oil pressure sensor fault from a P code indicates that the pressure detected is our of range, not necessarily a faulty sensor or low oil pressure.

Did you get a P codes? Use a OBD2 reader (amazon) to get the oil pressure code that can help identify where the problem is. You may also have to use the live data aspect of OBD2 to monitor the pressure.