@Minervamouse77 that's sounds stressful for you, sorry to hear that you're having to make these kind of decisions.
Have you read Expecting Better by Emily Oster? She has a brilliant chapter on the risks of antenatal testing so I'd recommend downloading it if you can.
Ultimately it's your decision as to further testing, if you would be happy to raise a child with downs and don't feel the risks of invasive testing are worth it you could just leave it as it is. The 20 week scan could show further abnormalities or a normal baby though.
From the sound of things you want to know for sure and the only way to do that is the invasive tests - CVS or amniocentesis. CVS is more advanced but can only be done between 11-14 weeks. They carry similar risks of miscarriage of around 1/600 - 1/800 (the 1/200 figure quoted by the nhs is wildly out of date).
This was all in the book and NHS screening booklets, I haven't got that far personally (first baby) but I read as much as I can about everything because I can't help myself!