BeeMyBaby, when we came back was out of the range of dates they prefer to scan, if you see what I mean. However, having slept on it, I came up with a suggestion that we go private for the anomaly scan. Not cheap (starting at about £200), but a damned sight cheaper than cancelling a holiday to North America, as insurance always insists you try to recover it through credit cards first, then of course there's the excess...)
I have spoken to the midwife, and she says they can accommodate scans done privately, and as for the confusing range of dates offered by various clinics I have looked at, she said every hospital or clinic had its preferred range. There was further explanation on the site of one of the places I was checking:
"The Anomaly Scan is best performed from 20 to 22 weeks however some consultants prefer that it is performed between 22 and 24 weeks because the views of your baby are often of greater clarity. The later time also allows the operator to perform some additional analysis.
"The reason the Anomaly Scan isn't performed earlier in the pregnancy is because your baby's internal organs wouldn't be fully formed.
"After 24 weeks a full anatomical examination may not be possible because your baby will often adopt a less helpful position. "
So that seems to be the clinical reason for the range in date-ranges insisted upon, which is extremely helpful to know, as "X to X weeks" can be said so dogmatically!