Update...
Yesterday we were still concerned. We took up the advice of talking to the ARC (who were amazing)and eventually made the decision to go to the Fetal Medicine Centre - a massive decision for us but I think the best one.
They did another test, which put my statistics for Downs into the 1 in c.3500 region: completely different from the results the hospital's sonographer gave me - obviously. I was shocked. They read the nt at 3.0mm, not the 3.4mm I had received the day before. I knew that took other soft markers into consideration but had no idea that the result would be so, so different. I asked time and time again whether the nt result was accurate....
I'm sure some may find this difficult to understand but we opted for the CVS anyway - my head could not make sense of the discrepancy between the two results and, I think largely because I had been so freaked out by what the sonographer at the hospital had said to me (see my post on the antenatal tests if you're interested!), I knew I would worry. Yes, the centre has an amazing reputation (and yes, I did believe that they knew more about what they were talking about.. by far) but I'm in a really, really good teaching hospital too: I felt I would be choosing to believe the result I wanted to hear.
So I'm now waiting results, which hopefully will come in tomorrow. It was a tough decision to make, particularly in the light of the new stats, but it was the best for us.
I've been left feeling very confused about the process of testing (how can the two tests be so so different?) and humbled by the experiences of those whose posts I have read in the antenatal tests section. To anyone who has received a less than happy result recently, my thoughts are with you.
Obviously am still concerned about the results but hopefully I will know soon.
Sincere thanks for everyone's advice and support in this thread.