Thanks so much ladies for adding your experiences to this thread - it has been a rare and precious source of information for me. I can't believe how little discussion there is about it on tinternet.
I lurked here in December and on the basis of what I read, booked the Harmony test at the Fetal Medicine Centre for next Friday when I will be 10 weeks 4 days. They offered it free when I booked the nuchal translucency test for £180. I owe you guys bigtime because I wouldn't have even known about it as it's not advertised on their site.
Incidentally Spire in Cardiff offer the Harmony test alone for £750!!
Even though we're in Bristol, factoring in £90 train tickets and then again £90 a fortnight later, the FMC is still massively better value.
I think the FMC are only offering the Harmony test free between 10-11 weeks as they're trying to build the dataset on which they can base the reliability - perhaps with a view to eventually getting it rolled out on NHS.
From the other stuff I've read about it, it's still a 'screening' test but with lots more data it might become the closest thing you can get to a diagnostic in terms of reliability without an invasive test, which is obviously like the holy grail for antenatal testing.
Based on the rather limited information I've found, it seems as though false negatives are not possible, but there is a tiny chance of false positive, which obviously can be checked for definite with amnio / CVS.
Has anyone heard of a false negative with Harmony?
I only have 5 more days to wait for the blood draw and viability scan (don't even know if the little anklebiter has a heartbeat yet) and assuming all okay (and not twins - test doesn't work with twins. Sweet Moses what if it's TWINS??) it will be a long old 2 weeks until the results of the blood test come back from the States and they do the NT scan.
I will keep you posted as to my experiences and the results.
Meantime, I'm wondering whether to cancel NHS NT combined test. If the FMC really is the gold standard, I don't want to worry myself (like Artigene's experience) with a potentially negative scary reading from NHS' inferior equipment and possibly less experienced sonographers. But then more data is better, right? And what if FMC don't get a decent sample or on the day, the baby's lying the wrong way for the NT?
On the other hand I'd prefer to limit the ultrasound exposure to the baby to what's absolutely necessary. Am broadly content with safety of ultrasound but have been slightly disquieted by some research I've read...that has not put me off the recommended tests but has definitely decided me that I won't be getting a 4D 'fun' scan. 
Would really welcome your thoughts. 