I paid about £500 and had the Harmony test package at the Fetal Medicine Centre. The NHS screening normally consists of the nuchal scan, plus some blood tests which look at hormone levels, plus a risk factor based on age, history, and lifestyle, and would have been very likely to come out as high risk given my age. I didn't want to risk amnio or cvs, both of which carry a risk if miscarriage, unless I was really quite sure of the risk. I also didn't want to have a later termination, and I intended to terminate if Downs, Edwards, or Patou were found.
With Harmony, blood is drawn at 10 weeks and cell free foetal dna is extracted from it. Chromosomal analysis is done on this foetal dna, giving a much more accurate result than the NHS screening. The NHS is currently trialling this method in some parts of London, and it's likely to replace current NHS screening. You will receive results from Harmony 2 weeks after the blood is drawn (by 12 weeks if blood is drawn at 10 weeks) which allows you to terminate quite early if needed.
The Fetal Medicine Centre donates all profits to a research foundation for fetal medicine, and the package I had included a detailed viability scan at 10 weeks, Harmony test, and detailed nuchal scan at 12 weeks. If you would terminate for Downs, or for conditions incompatible with life, and you can afford it, I can't recommend it highly enough.
Fwiw, all my results were extremely low risk and my detailed scan showed absolutely no abnormalities. It has allowed me to actually relax and enjoy my pregnancy, as I feel like now I have taken all sensible precautions and anything that might go wrong now is out of my control and also very unlikely.