We have a double mattress free to go! It's a double (135x190) Slumberland with a PostureFlex spring system. It was used in our guest room.
You would have to come and grab it from us (Crouch End)
I was just about to organise a council collection but then I thought that it's too good to end up in the skip.
We are on the second floor with no lift but I'm sure Markypoos will be happy to help and the mattress as it's really not too heavy!