Right, so after fiddling about with muslin, pins and shears, I was finally happy with my toile. I only adjusted half as I thought it was pointless fiddling about getting the other half exactly the same. I then carefully undid the stitching down the back middle seam to separate out the altered half of the toile and transfer it to paper. I used a combination of the original pattern (for accuracy) and the muslin toile to mark out the new pattern pieces. I had a dart on the muslin toile, but this nicely laid itself out flat on the paper, so less hassle there.
Then all marked out, it gets cut, et voila:
See you soon fellow crafters