Update - Monday Evening.

Note: Several people have suggested the transdermal gel for the thyroid medication. I asked about this when he first went on the pills but it is not available in the UK. I also checked online, not because I doubted my vet but just in case I could find somewhere that does it that they did not know about. There is a compounding laboratory, but the thyroid medication is not licensed to be compounded into a gel. I also looked into the possibility of having it shipped here but that can only be done under special license. 

Thank you once again for all the purrs and helpful suggestions for Flynn. He is spending all his time either on the chair in the sun, or on the bed wrapped in my furry dressing gown.
The last update on Saturday I said he was eating a little and I was cooking some fish for him. He never touched the fish or ate anything else that day. What started as one of his good days went downhill very quickly. He seems to get one good day then one bad day. He stayed on the bed and looked very ill. I don't think he was in pain, but we all know they are very good at hiding it. He was in obvious discomfort though.
We talked all through the night and said we mustn't keep trying for our sake when it was not in his interest, and the bad days were getting worse each time. We intended ringing the emergency vet in the morning. Yesterday morning there was a very slight improvement so of course we didn't ring the vet. We want to give him every chance to recover if it is at all possible.
We collected some more Zantac injections and also B12 which we gave immediately and also an appetite pill. Later in the afternoon he started to improve and even ate a sardine (thanks for the suggestion Bonnie).
Today I rang the vet who has been treating him and we took him in for another blood test to see if anything different is showing up yet. We are expecting his T4 to be raised as he is off thyroid treatment for the time. He is to continue with the Zantac, but now in pill form rather than keep injecting him twice a day. We brought home some Hill's Prescription AD diet too. He has eaten 3/4 of a tin, and also one of his small tins of food as well. They weighed him and he has lost nearly one and a half pounds in 10 days so it is very important to try and keep him eating one way or another. He is so very thin and frail, but still so full of love.
After all this doom and gloom the good news is he has done a firmer poop yesterday and today.
As long as he is able to keep fighting and has a chance to get better, we won't give up on him.
Please keep purring for our boy, he needs them to help him through this.