Sad news guys.
This morning I got up feeling better about Kenji. Yesterday he was a lot more responsive, swimming around and coming over to me when I came up to his qt container. His ich was disappearing fast and his finrot was nearly gone, he was even having a bit more food.
But this morning I found him on the bottom

I scooped him out and saw that his gills had puffed out. I'm guessing that's where the ich had gone??
I spoke with my mother and we agreed that when my sister got him he must have been fairly old already. My sister said he was making bubble nests in his little betta tank on the second day and that he hasn't grown much, if at all, since she first got him. He'd been with us for just over a year so he might have been at least 2 years old??
Anyway, I'm going to give everything a good scrub then have a good look around for a new fighter. Possibly a crowntail this time.
Thanks to you guys I now know what to look for and how to look after these guys better, even if it was learning the hard way.
Here's a pic I took just after my sister got him. As you can see, he hadn't grown much