Here are the pattern pieces I came up with, the top one for me and the lower for James:
For my first pair I just cut out the original seat and sewed in the replacement piece to occupy the same place in the original. I cycled in the fixed pair for a week. They are a bit tight. It seems to me to be not a good idea to have the fabric under strain as the new strong cloth may cause the weaker cloth to stretch further. So for the other two pairs I adopted a slightly different strategy, pinning the replacement piece onto the garment while they were being worn and making sure that the original fabric was not being pulled out of shape. This means that the replacement piece is fitting in a smaller space than would be expected from the pattern. Nevertheless it seems to work. We wore them today and they are comfortable. Time will tell if these fixes will really work. If not, I already have sufficient Powerstretch in the cupboard to make more.
This is, of course, in the back of my mind:
Mark 9:16 "No man putteth a piece of new cloth unto an old garment, for that which is put in to fill it up taketh from the garment, and the rent is made worse."