Thanks!
Update on the chicks - they have had several nights outside now and are doing fine.
I got a lizard heat mat (couldn't get a brooder one) and just happened to have a copper plate picture my Nan bought from a charity store about 20 years ago. I taped the mat to the copper, put it in a bag, wrapped it in newspaper et voila, underfloor heating!
I also had a hot water bottle in a fabric bag which the chicks tucked themselves into so they have been completely fine. Weather is warmer as well.
Next challenge - how to stop my cat from chasing them! He is currently throwing himself against the wire trying to get in.