When my midwife told me she had requested my first scan from the hospital I actually called the hospital a couple of days later and asked for a specific date as my DH had to go away (which was all true), they literally scheduled the scan for the date I requested!
For the 20 week scan I was told by the sonographer to call 3 weeks before (so at 17 weeks) to book the scan, again made the appointment there and then when I called. I much prefer this method, than having to rely on my MW to arrange everything for me, and also means you can book a date that you are definitely able to make.
Might be worth giving the hospital a call direct.