Well, I don't quite have the definitive answer - I've had the scan this morning, but consultants appointment to interpret result is now next Tuesday!!
Scan result shocking though, gone from placenta still touching the top of the cervix less than 4 weeks ago to now being 3 cm away!!! (And her head is below the leading edge of the placenta, which I guess means she can come out before it) Having not seen the consultant obviously I don't know the official answer but my understanding is that over 2 cm away and they let you get on with it yourself....
I'd better get back in the frame of mind that she could arrive any time in Aug or Sept 
And she's put on about a kg in weight since the last scan... No wonder my back is starting to complain!