I'm reviving this thread to update and hopefully get some hints as to what may be happening.
Things have changed since the original post, for a variety of reasons and without rattling on for pages with all the details, the upshot is this.
Molly is now eating raw in the morning, Nature's Menu, nuggets (Hunters complete rabbit, venison or lamb, still avoiding chicken at the moment). This food contains 80% raw meat, minced bone and some veg and fruit. This she loves. She was previously having JWB kibble in the evening but over a period of a week or so, this was switched to Fish 4 Dogs kibble which just contains fish, potato and peas. This she also loves!
However, and this is my reason for posting. Since the switch to Fish 4 Dogs completely, which is just the past few days, Molly has been producing a prodigious amount of poo

Yesterday she went 5 times, each time a substantial amount, solid to start and then a bit sloppier toward the end of the day.
I'm actually feeding slightly less than the guide amounts for a day, obviously halved in both cases. Having read back through the various posts in this thread, I'm beginning to wonder whether Molly is not managing to digest one or the other of these meals and I'm going to have to change things again.
All this in the hope of unblocking her anal glands and avoiding the various itches - what a minefield this is

Any further thoughts and opinions would be much appreciated.