I didn't get the NT scan done, but like other ladies I had a dating scan done at 12 weeks. This meant I got to see DD, and the sonographer had a quick check of her nuchal thickness and nasal bone anyway and said that it all looked good.
If the sonographer's are looking at nuchal thickness, then they are doing a screen. This is ok if you are happy for it to occur and it all looks good, BUT if you have stated that you don't want the screen (and don't want the info that measuring the NT can bring) and this is done then you may be presented with info that you didn't want to have.

All ultrasounds are done to look for things that are 'out of the norm', and people should be aware that this is their real purpose, not to give parents an opportunity to look at their baby.