I had mine at 19+4 but that was because I was booked in when I was leaving the hospital from my 12 week scan. They gave me a choice of a couple of dates, so I got to book in the easiest day for me to get there. I was surprised when I got home & realised that I wasn't even going to be 20 wks by then. I was expecting to be given a non-negotiable date in the post like you do for most hospital appointments. In our area, however, the booking timescale seems to be between 19 & 21 weeks.