daisybob, sorry to hear that you have had a scare at your scan.
I have come across this as a cause of some interference with the growth of limbs of babies, but I am afraid i can't remember exactly how it works. I have read about it on a parents site called Limbdifferences.org If you can't find it, post again and i will find the exact address. It is a busy, freindly American site (but lots of u.K parents too) for children with limb Differences.
I think that Amniotic banding can SOMETIMES cause loss of fingers, or part of an arm or hand.
If this turns out to be the case, please keep posting. I am a Mum of a DS who has an incomplete foot a short leg and a bone missing from his leg. This was obviously a bit of a shock when we found out (at 20 weeks), BUT the preparation before his birth was SO reassuring, and we came to know that it was simply a structural thing, it doesn't threaten his health, it certainly isn't affecting his 100% enjoyment of life, and has not for one nanosecond interfered with our joy in him. It just isn't an issue.
Please keep us in touch about this, daisybob, and do feel free to contact me off board if you would like to.
But remember: it's just a maybe anyway.
Hugs and Good luck.