Following my nuchal scan, the midwife wasn't even going to give me the measurement, saying it was policy to just tell me a combined risk figure! Anyway, I read it over her shoulder and have done a lot of googling. I have found this on the "National Center (yes, it's American) for Biotechnology Information" based on a Korean study.
Oh, and of course, it's not a substitute for medical advice, talk to your own midwife etc etc. However, I found the most frustrating and patronising thing to be the lack of information and it seems that the question "what is a normal NT" often comes up on threads here an elsewhere. So for those who are interested:
CRL = crown rump measurement in mm. The 5th / 50th / 95th figures are the percentiles - apparently anything over 95th percentile is deemed high risk. The numbers under the percentile columns are the NT measurement. Sorry it's difficult to read, but posting data from a table is quite hard!
CRL 5th 50th 95th
40 0.31 1.22 2.14
45 0.40 1.32 2.24
50 0.50 1.42 2.34
55 0.60 1.52 2.44
60 0.70 1.62 2.54
65 0.80 1.72 2.64
70 0.90 1.82 2.73
75 0.99 1.91 2.83
80 1.09 2.01 2.93
85 1.19 2.11 3.03