My daughter is at St. Mary's and I suppose headed for Harrytown but we too are a little concerned by the Ofsted report on the standard of teaching. Also, other parents with older children at Harrytown have reported poor communication from the school and rather too many supply teachers. I don't know that there is any alternative however, other than Marple Hall which although seems to be huge, does seem to be well resourced. I know of one parent with a child going to Poynton rather than Marple Hall but logistically that's not ideal.
And no - don't take Ofsted reports at face value. For example, since that one, several teaching staff at St Mary's have left or retired so there are probably only going to be 2 left out of the 7 who were actually teaching at the time of the inspection!