var inner = from x in TableBody
            group x by x.id_Header
            into g
            select new { g.Key, Summa = g.Sum(y => y.kol * y.cena) };

var query = from x in TableHeader.DefaultIfEmpty()
            join y in inner
            on x.id equals y.Key
            where x.id_C == 10
            select new { x.id, x.Date, y.Summa };