I posted a couple of weeks ago about two feral kittens and their Mother who were living on the train line near my house. I got some very helpful advice and just wanted to give an update in case anyone was wondering.
We got in touch with someone from CPL who advised us to feed the cats at the same time for a couple of weeks to get them used to us with a view to catching them.
Sadly in that time one of the kittens was killed (we were told about this by someone who saw a man taking the kitten away, apparently s/he was run over 
)
We got the trap today and DH spent an hour outside and managed to trap the remaining kitten (who is now known as Topsy 
)
Topsy is at the vets now, they will figure out how old s/he is and neuter if Topsy is old enough. We'll pick Topsy up in the morning and keep him/her in a separate part of our house to our cats whilst I convince DH to let me keep him/her we try to find him/her a loving home.
We'll also get the trap back from the vet tomorrow and try to catch the Mother cat to get her neutered.
Thank you so much to all the posters who advised me on the last thread!