I made the appointment for the 20 weeks scan when I was at my 12 weeks scan, no more letters.
Although I do remember that when I got the letter asking me to ring for an appointment for the dating scan, it said "You've got this letter either because of the 12 weeks or the 20 weeks anomaly scan, please ring bla bla bla".
Did you maybe just a one size fits all letter, which reads a bit ambigously?