Mine says usualy within a week to phone, otherwise they will send a letter out within 3 weeks. It's been nearly 3 weeks so as I haven't heard from them it's obviously low risk. Just worried I'm that one who has been missed. With my son last year I had the letter back with the % quite quickly, so maybe they are just busy.
Had the bloods done a week before the scan, then results got plugged into the computer on the day of the scan and given to us there and then.
Have you had the nuchal fold ultrasound yet? If not, they're probably waiting for that. If not, and you've had scan plus bloods and have been waiting over a week or two, you could follow up with a call? Things do go missing in the post etc, but as others above say, no news is good news in these cases.