Thank you to everyone who commented on my last post about these - I'll give some more details about what I've done in the next couple of days, when I've actually finished them!
In the mean time, just look at the improvement I've already made.
My next step will be to make up a second toile. I'm going to add a small amount to the width of the trousers, roughly down the line of the centre crease. I'm hoping that will releave some of the tightness across the seat. I'll also deepen the crotch curve a little more, as doing this a bit has helped a lot already. Then I'll go back to looking at the darts. The shape of these is already so different to the pattern piece it's a wonder they fit at all to begin with!