A very quick update to report the phone back I've just had from Rowan.
Unfortunately the very helpful lady from Rowan didn't have the actual garment with her to be able to give me the tension the edging was knitted at but she did have some interesting measurements and statistics.
The garment in the pattern book was the first size and took 660gms. Two pattern repeats fitted along the base of each sleeve and I think she said 7 repeats along the bottom. The edging measures 29cm wide (ie parallel to the cast-on edge).
My edging only measured 27cm so her conclusion was that I was knitting too tightly and that if I used larger needles all my troubles would be over. I've now restarted and so I'll see if this cures it. I feel rather sceptical but lets see.
It certainly would have been helpful to have a tension given for the edging.