Thanks for reminding me Cyrus. I'd forgotten to report back.
Well I decided to try one more thing in the non shoeing arena before resorting to putting shoes on her because I really have an aversion to shoeing if it can be at all avoided.
Anyway, I thought long and hard about what Di J had said, read all I could find about high/low heel syndrome. Looked at the musculature involved and went surfing the net. I ended up with an idea to try which was basically to allow the sole of the hoof to direct the trim, ensure the frog is in full contact with the ground, not trim the frog at all bar loose flappy bits and most importantly - I used a spirit level to measure if her styloid processes were level. That last bit took a fair bit of work. I have been trimming her hooves this way since just before Christmas and I have to tell you the results have been amazing!
Di was out on Friday and she was astounded to see the changes. Firstly, you can SEE the hoof growing strongly from the coronet in a different, more appropriate angle on BOTH feet. Secondly, ALL the muscular issues that were indicative of the high/low heel syndrome and that had been there for many, many months previously were gone. Completely gone.
So I'm going to keep up this trimming regime and see what happens.