Um, yeah, I had that zip unpicked and redone within an hour of my last post! Natasha was right - it would bug me every time I looked at it. Thanks everyone for the helpful comments. The seam isn't on the true bias, and being a back seam it has a curve at the waist, further complicating things. My wedding dress was also stretch charmeuse cut on the bias, and I had to do the zip a couple of times to get it right. Fortunately I remembered yesterday what I did - I stretched the fabric as far as I could as I applied the zip. You'd expect that to make the zip very ripply, but it actually counteracts the weight of the skirt almost perfectly. I took a very hurried pic after I finished but before I pressed it. (Twins were waking up and sewing time was over for the day, sigh.....)
If you click on it you can see it bigger. When I put the pic on the computer I realised that one side looks fine and one puckered, but I swear both were smooth when I tossed it on the dummy! I think it "caught" on the fuzzy dummy cover as I carried it outside. Anyway, sleepless night averted, and I'm now happy with the zip. (Or I will be after I press it and check it is smooth on me.)
I left it on the dummy overnight to let the bias drop. Since the fabric has elastane in it it is less susceptible to dropping, but I'm still expecting to have to even out the hem. Today's job is to finish the neckline and armhole edges, and then I'll leave it on the dummy overnight again before hemming it tomorrow.