I reckon run with the excuse only one person can come in for the scan Nellebelle. Having had a few of them myself, there is only ever one chair in there so just say yep its on this date mum and DH is coming. Dont tell them you are getting a video as you normally get given a couple of still pics too and I am sure they will be happy looking at those. She may not ask and if she does just say sorry mumsie, only one person can come in as the room is tiny etc..
Goodluck!