Hi everyone! Just thought I would update on this thread because loads here were so kind and helpful.
We eventually got the kittens out of the fireplace, I had to rip it off the wall though so I'm hoping it can be fixed soon 
They were all weaned and litter tray trained and that was really easy with this litter. They all just seemed to learn from each other very fast and I didn't once have to pick up kitten poo off the floor like I had to with the last feral litter.
Anyway when they started exploring and getting stuck places that weren't safe I decided it was time for them to go to their forever homes for some proper 1:1 time with humans, especially pepper (with the string) as she was very nervous and getting aggressive.
Apparently every single one of the kittens has settled in beautifully! All of them are happy and purring and cuddling and playing! I think maybe they had some sort of feral gang mentality going on.
I've received photos of 3 of the kittens in their new homes, all looking so happy and content!
I've only let them go with the deal that if they have any issues whereby they don't want them, they would have to come straight back to me and not go anywhere near cat prison. But I really don't see that being an issue with the owners I have chosen.
I would like to say a massive thank you again to everyone who helped me. And a special thank you to their new parents, particularly peppers mum who wanted her even though I said she was particularly feral and would need a lot of work and asked for advise from professionals before collecting her so she knew just how to play it.
The other tabbys mum (Delilah) was such a lovely person and DS quite liked the fact that she distracted me with conversation so he could go and pinch some cake and try to get into the box of chocolates she brought 