You don't mean you've kept the new plants in the pots do you ?
If you 'do' they are probably too dry - and have you checked that they don't need repotting ?
They're going to need massive pots if you're thinking of keep them potted.

If they're in the ground and they've been planted in that builders rubble they'll die because there isn't the soil, nutrient or moisture there to nurture them.
These trees are thirsty and need good soil and lots of water.

Maybe they are competing for water?

Probably true, Drc. Also, dead wood underground, including the roots of the old trees, within 1/2 meter of the new plants can often cause them to suffer.

Thanks for the responses so far. The new trees were planted directly into the ground [not in their pots!] after removal of as much rubble and old roots as possible. They have been watered regularly but not fed. What feed does anyone recommend?

I have one of these growing in a pot, had it 8yrs now.. I use a slow release feed for mine once every 6 months and water it every day in the summer and mine is flourishing. I have recently had to re-pot into a massive pot, it was very happy I think as it was a bit crammed in it's last pot lol.. I have cut mine so it looks more like a tree so not any branches on the bottom foot and it's about 5ft tall, I love it, I am very happy I kept it after many people told me not to, they are beautiful trees. :)

PS, mine has only every gone brown lower down and on the tips when I have neglected to water it in the past, that's how I ended up with the lovely shape I have now as I had to prune of lots of brown in the early yrs so I would try extra water, mine easily takes 4 litres of water a day in high summer and at least 2litres a few times a week in the winter, I am pleased to say my conifer is well cared for now and no longer neglected lol, I added lots of sharp sand and stones to the bottom of the pot when I re-potted recently to help retain water and my conifer has thanked me for it.

Never ever going to be easy to establish replacement trees (conifers) within an existing hedge at the best of times, lack of nutrients, water, etc. Replacing a lot of the soil in that area would be beneficial if possible, plus a lot of ongoing TLC and hope for the best

Thanks for the follow up. The conifers seem to have come through their early problems and following much watering are now settled. Some of the lower branches are a bit brown but there is plenty of healthy new growth.

Good news there then. :)