I have a rather large concrete floor (26sq mtrs) that has a few dips in it and I need to level it with self level compound, do you need or can you supply a retarder admix to make it workable for longer so it doesn't go off too quick and i end up with another uneven finish. I need it flat as were putting electric underfloor heating in. I have bought products from you before, satisfied customer. Regards, Mike C.