Thank you so much for the help.
We went to see the vet at the hospital yesterday as our normal vet is on leave and her replacement was frankly condescending and rude. He also seemed to have absolutely no idea about how to manage FD or such a complex case.
The vet at the hospital looked after him during the DKA.
So bloods came back - pancreas was tested and in normal ranges. Potassium low (starting kaminox), liver ALT still very high (500) and urea was high (but dehydration suspected).
The hospital vet discussed where we could go from here. We didn't feel another £2-3k of diagnostics was really the way forward here (£600 in vet bills this week alone -£5k during DKA and he's not insured) and she also agreed probably not going to find anything since they did most of these tests only 3 weeks ago to no result. She suggested we try B12 shots, antibiotics and she's given some antinausea pills (he has been nauseous after eating - no vomiting yet though), along with Fortiflora. The concerns here are really that with something going on in the liver, potentially something with the kidney and newly discovered yesterday was a galloping heart beat, what connects these things? It's heartbreaking but I can't put him through much more.
So we have started him on that lot and see if we can make some improvement and go from there. Florastor arrived and I started him last night (concurrently with fortiflora, I figure another bacterium can't interfere).
I don't want to 'jinx' it, but overnight he only produced a tablespoon or so of semi formed business, so that's the best situation in 3 days, he seems a lot happier this morning so we must once again 'wait and see'. Sadly he's stopped eating almost anything except canned hills m/d now, which he hoovers with great relish. It's not ideal but at this point I just need him eating something and keeping it in long enough to get nutrition.
Vet also explained insulin resistance when you have inflammation, which tallies with why yesterday and Tuesday although he was shot correctly produced almost no discernible drop in BG (these were his 2 worst days).
AMPS 26.6 - shot 2.5.
Once again, thank you for the resources. This community of strangers is just incredible, I'm blown away by everyone's willingness to help. I don't think he would have made it this far without you all.