Bad install - Front door

I had a 4000 EUR front door installed by a reputable company from Leuven this year.
During install I questioned their way of working, the door wasn't installed flush against the existing wall, it left a gaping hole of around 10cm (with they later filled with cement, looks ridiculous!). They argued that it was within specification. I signed it off in the end.
The more I look at it and now that winter has started I realize it's a terrible installation and that it's can't be in spec, a total rush job.
Where do I stand legally as I can't leave it as it is.
Thanks in advance for your input.