Drainage?

Since demolition of a school on land backing on to our property we have had flooding in our back garden. The builders said when they install the drains there will be no more flooding. Wrong it still floods. There is now a gravel filled trench along the backs of all properties involved but still we have floods, they say its a 'French drain'. What's that? The ground of the site is around 1 to 1.5 m higher than our garden. Q: The talk is now a ‘Soft Drain’ what’s that?
which they will construct in our garden to run to a suitable drainage point nr the house. I believe this could cause problems with out existing surface water drains by the volume of surface water coming from the site in heavy rain and could either burst our drains or even lift lids on manholes. I am very concerned, if we agree, as there has been no menton of maintenance/replacement resposiblitiy, after all we did not suffer flooding when the school was there, and the council seems useless on this matter. Any thoughts would be most welcome. I appologize for the length of this.
Many Thanks.