I did it at 12 week. Good, as it was a senior doctor who did the ultrasound for me and he explained very detailed the body structure of the bb and even told me the sex of the bb.
I did it at 13 weeks. I think it is worth to do that !!
Also it is always fully booked, so if you decide to do it , call them as soon as possible. the test should only be done on 11-14 weeks