Thank you all for your suggestions and tips! Having taken an age to rip the first seam out, I've pinned it all with so many, many pins (pinning seams open - my machine seems okay with the thickness - it's the transition between seams and not which I am finding tricky, so I have gone really slowly and carefully with the thick seam tool, and using my walking foot). It seems to be going okay. Oh, and I made the seams a bit wider, to make absolutely sure they'll hold. Slow going, and I think I may try the middle of the sandwich smaller next time, but I think I am getting there now, slowly but surely. Thank you again, I really appreciate it!!