It is good that she is at least eating
something for the time being. Other foods might be upsetting her tummy, or she may just not like the taste. I feel your pain over the trial and error to find a food that agrees with the cat (been there).
Some cats will lick the jelly off food and only go back for the chunks later on, but that's no help at all when you're trying to treat a diabetic! Have you tried pouring some of the tuna water over the pouched food and mixing it in a bit to see if she'll eat more for the feed at insulin time? Also, some caregivers say that their cats like Bonita flakes as toppers on their regular food (not sure if you can get similar in the UK).
Another thing that might be worth trying is to crumble up some freeze-dried protein treats over the food. I've had a fair bit of success with that. Sainsbury's do Thrive pet treats if you want to give it a try (and I'm sure the likes of Morrison's and independent pet shops in your area may also carry them). If it works then you can get less pricey Cosma Snackies freeze-dried treats from Zooplus:
http://www.zooplus.co.uk/shop/cats/cat_treats_catnip/cosma_cat_snacks
They're available in chicken, fish and tuna flavours. The XXL ones are a bit cheaper and you can break them up into smaller chunks. I use the chicken treats and blitz up a handful to make a 'chicken powder' to sprinkle over the wet food, but you can actually crumble them into a powder with your fingers. I've not tried the fish or tuna varieties so don't know how easy they are to crumble.
Some caregivers swear by Fortiflora as an appetite enhancer. It's the same sort of stuff that they spray on kibble to make it palatable to cats. It's not cheap (try animeddirect.co.uk - usually have the best online prices) but you only need a tiny sprinkle and a sachet can last for a few days. Word of warning: Fortiflora contains probiotics so it may not be a good thing to give to a cat with any history of pancreatitis. Also, not all cats like it and it can make some kitties nauseated.
Mogs
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