Yes part of me agree's with what you are saying you will always have in the back of your mind what if and is everything ok it doesnt matter how many u/s & tests we have us mums who have lost bubs will always worry.
But you also have to ask yourself what if you get to the 16 weeks with no more u/s or your nuchal scan and you get to 20 weeks and there is something wrong what would you do? would you continue a pregnancy with an serverely abnormal baby if the answer is yes then by all means relax and enjoy your pregnancy.
Myself i have had to end a pregnancy with a serverely disabled child with no prospects of a life and it was terrible i have had every test available with this bub u/s CVS and amnio i had to be sure i could not go through that pain again i would rather m/c early if nature intended then go through what i had to.
But i hope what ever decision you decide makes you relax and feel more optimistic that this pregnancy will be a healthy one for you.
I know what you're saying and to be honest I don't know. In my mind I would never terminate but I've also never been put in the position where I've have to make that choice so can't know for sure.

Surely they would have an idea if something was wrong if I had another u/s around 16wks?

My biggest fear is having the u/s's most of my m/c's have happened within days of having an internal u/s. I Know it's silly but most of the time I've had one I've been told everything is ok then days later start bleeding and its all over.