Thanks for all your advice and support. After discussing it with DH, we are going to have the amnio, just so we know either way. Otherwise I will stress about it for the next 6 months.
As for what we will do if it is positive, I don't know. If it was my 1st or 2nd child there would be no issue, but this is my 4th, and I don't know if I can manage the older 3's needs as well as the extra demands of a disabled child. But at the same time I don't think I could electively abort a child, especially since people with Down's can lead a relatively normal life. But that decision is for later on.
As DH said, we have a 6-7% chance of Down's, so a 93-94% chance of a normal baby. I have to focus on that, the odds are in our favour.
Thanks again for your words of support.