Didn't know whether to post, as it didn't have a satisfactory conclusion, but thought I would just quickly update.
They're still using my shed as HQ but I do see them with mum leaping over the silage bales on the farm, etc. so they're getting out and about.
Despite the farmer's sister's rant I have continued to feed them and they seem to have doubled in size over the last week. They're like miniature cats now, not kittens! I've fed sprats, dry food, butchers choice, value food, duck carcass (minus breasts and legs) - just a mix really.
I can stroke them all. The two torties are shyer, but the ginger and white boy is an absolute darling. I can pick him up, cuddle him, he purrs and lies on his back in my arms.
Even better, all of their eyes are looking healthier. Not perfect, and I know a responsible owner would still get them treated, but they're definitely looking better. So, while not an ideal situation, I'm still happy that they're all alive and nourished. Here is a pic, poor quality sorry!