I rang the geneticist again yesterday and he did ring back today. The result of dd2's Rett syndrome test arrived yesterday (how convenient after 9 months delay!) and it is negative - a big relief after such a long wait.
It doesn't seem likely that we'll get a diagnosis now as this was the last idea on the list, but I'm not too bothered. At least nobody has any expectations of what dd can or can't achieve. She is doing quite well at the mo - commando crawling about a foot towards interesting objects and dragging herself towards me to give me a hug!