Oh, no, I so sorry you have this added stress jaffa of a 'high risk' NT result (Incidently, DS2 did the hands into the nappy into the poo thing for the first time yesterday, too... luckily, I had DF on hand to help)
First of all get a break down of your results - the nt measurement, the hcg/papp-a bloods. Did the scan look for any of the other markers indicated with down syndrome (nasal bone, the 'easiest' to spot at this time)?
You will no doubt be talked through your options - which as far as the nhs is concerned will be cvs (sooner than an amnio, but riskier), amnio (later than cvs, but less risky), additional scans specifically to look for heart problems (again, later, and not diagnostic as such), or wait and do nada (If you feel you wouldn't terminiate for down syndrome then you could opt for an amnio at around 37 weeks for a final yay or nay answer that doesn't put the baby at risk if it does start labour... but would more than likely give you a chance to prepare should it be positive).
As you're probably aware there is a dedicated board here on mumsnet for antenatal screening results/choices and I believe the charity ARC are recommended as someone to talk thing through in an unbiased and non-judgemental way
When it's not happening to you, 1:20 isn't really that bad - as already said, statistically, things are still very much in your favour...
When it IS happening to you, I can imagine your fear, panic and sadness - and I'm hoping with every fibre of my being that you are one of the 1:19 who would have a GOOD result.
You may remember that I came out of my nuchal scan so worried that I went to the fetal medicine centre in Harley street the next day. I can't remember whether you're near London or not...
You can do this, we'll get through it one step at a time.