Installation problems

Window installation problems. We used a company registered in Belgium to put in new windows. It has been a series of mishaps. Two of the windows were installed with rounded inner edges, and the company told us that it would not possible to find blinds to put on the windows. Later, they came and told us that they would use the blinds from the old windows, but instead they put new paper thin blinds on of the cheapest quality. They ignored our instruction to replace the windows and blinds with blinds of the same quality. The windows are south-facing and are where the children sleep. In the summer the rooms heat up like a sauna, so it is really essential to have blinds which block out all the sunlight. This was not done. They took away the blinds and came back several weeks later with not quite paper-thin blinds that still let in a lot of light and which are not of the quality we explicitly required. These were installed by taking the windows apart and putting them back together again - that is removing the four pieces of outer casing - frames - around the glass and putting them back together with the new blinds, replacing the upper piece of casing where they drilled holes to put in the first set of blinds. They tried to do this without our knowledge. They told us that the blinds we asked for could not be fitted on our new windows, as the new windows have rounded inner frames, unlike the old ones which were not rounded. We at no time asked for or approved windows with rounded inner casings, and as this is the stumbling block, we feel it to be within our rights to demand new windows to replace the (newish) windows which were installed and which now have gaps between casing and glass.
The question is, where do we stand? Can we insist on having the windows replaced and put in as we asked initially? If this does not happen, will the windows be guaranteed by the installers or by the producers - a separate company? Will the producers still honour the guarantee, now that the windows have been taken apart and put back together?
Is there a consumer association or advice centre in Brussels or elsewhere in Belgium that can answer these questions with assurance?