Just wondering what the window fitters on here do in this situation. I'm fitting a large R9 triple glazed unit 2100 wide x 1400 high. But I notice the in situ cill is bowed i.e. high in the middle, with a 10mm gap each end if you put a straight edge level on it. I ended up replacing the cill on the back of the house as that one was in pieces, it was a real ball ache making the mold etc......and as this one is big I'd have to make it in 2 pieces. I was thinking of trying to fill the 10mm gap to get a level base for the window. I'm guessing the norm would be packers and foam with silicone at the front to conceal?