I think you may have to teach her to use the cat flap. A lot of cats don't realize how they work. Being in the States, I'm not a fan of outdoor cats; but I realize that you UKers do that all the time.
I'm sure your little Sebastian came by to tell you he's OK. So many are there to take care of him.
I'm so sorry for your loss. He knew wonderful love and care, even if for a short time.
Two of ours had very bad diarrhea. We gave them canned pumpkin (not pie filling, plain pumpkin) mixed in their food. Flavorless metamucil (not the orange flavored). It's pretty hard to find. The vet gave them flagyl for a while. I switched them to PetGuard food. One ate chicken and wheat...
It is very awful. We can't leave anyone at our regular vet overnight. They have no overnight staff, so we bring them home. We can leave some at the ER and have had to. That's the worst because if they are there, they are very very sick.
Loads of luck to you and them. I'm sending good thoughts.
I'm so very very sorry. I know how much this hurts, but you haven't failed anyone. Your sweet Gizmo knew a great deal of love in his short life, and he went to the Bridge warm, well-fed and well-loved. Try to think about the happy times you had with him. I hope your heart can find peace in time.