@Becky did you have one? I never had one with my first so curious what it entails 😂 x
I had a failed one at 40w (they couldn't do it as my cervix was completely closed) and a successful one at 41w where baby came 14 hours later. It's a very quick procedure where they put a finger through and swirl it around basically where the baby's head is to lift the membrane away slightly. A bit uncomfortable but fine! They only work if you're body isn't far off labour anyway - I reckon my boy would have come on his own maybe 3 or 4 days later, but they're a good option if you're getting close to too overdue or are just suffering in late pregnancy. It does come with (low) risk of infection or your waters accidentally being broken though, so be sure it's what you want :)
They should be fine to do it at 40 weeks :) I didn't have to book mine, they just asked if I wanted one at my 40w appointment x