footle thank you for checking. finally got a moment to myself 
DD saw physio who happens to be related to someone we know so put her at ease straight away, a lovely woman, spoke directly to DD most of the time, with me confirming/backing up/elaborating/filling in gaps.
She said that all children are to some extent or another hyper mobile in some joints, due to the constant growing and stretching. She doesn't think that DD's current problem will be a longstanding hyper mobility problem, and also that there is no issues with the alignment of her hips etc. But it's too early to tell for sure.
She said that her calf/back muscles are very tight, and the muscles at the front/shin are weak, in particular on her right side, and that is very likely because she has had a particularly big growth spurt, said that children will often have small periods of aching legs, which is 'growing pains' as the muscles take a little longer to catch up, and some children (i.e. DD) have more problems with this, that the bone growth vs the muscle and ligament growth are significantly different for a short while. If not addressed it can cause problems later as the muscles can remain weak/tight for too long, but it's rectifiable now. The instep is very pronounced on her right foot in particular and she said that the reason she can twist her feet all the way in the opposite direction is due to the lack of strength in the muscles.
Also has weak hip muscles, for the same reason, but that many children have this, and when they exercise they don't do specific exercises to address specific muscle groups, children just do lots of fun activities, unlike adults who build specific muscle strength.
So, 4 exercises to start with, to build hip muscles, and ankle/leg muscles, to stretch out the calf muscles, and to help with encouraging her feet to turn outwards. She said there are more, but that she doesn't want to add too many as if it causes more pain than good she can tell more easily what is working and what isn't.
There is a group class in the gym with children, with all different reasons for being there, and said she thinks DD will benefit from this if she gets on with the exercises. but if they hurt too much, she will do the exercises with her in a hydrotherapy pool.
She assured DD (and me that there is no actual damage to her muscles or her ligaments, and although her legs might ache afterwards, the exercises are not doing her any damage, and it's going to take a little bit of time, but that she will feel much stronger in time.
Advised for her to keep swimming, and to not avoid activities where possible, but be guided by her pain levels, particular over the next couple of weeks as she starts the physio exercises. Said she should do PE and stop early if needed etc.
Back in two weeks to review and build on the exercises.
She said that further down the line we will have a clearer picture of whether this may be a longer term issue, but she thinks it's likely linked with growth.
She also mentioned hormones, can't remember who said that! And she is definitely having a surge of hormones atm!
Thank you all so much. I will come back to this from time to time to remind myself of some of the info you have all given me, re shoes, tips for managing the aches etc.