Peter, knowing that these are container grown plants opens up a long list of possible problems.....with the potting medium itself as well as the growing and cultural conditions your trees are exposed to.

Perhaps if you could share some more details we could help. I sure wouldn't give up on the trees if the problem can be fixed.

I'd like to know such details as the size of the trees, size of containers, ingredients of the potting mix, fertilizer type and application details, watering habits, etc.

The size of the pots in question are 24x20x18 which I believe are quite large enough as the tree is only 6ft high.The compost used was John Innes no6 and I use a water meter prior to watering.As to fertilizer I always use phostragen as its a good all round feed which is applied fortnightly as they are in containers.I hope this gives you something to go on and thank you for the reply.
Peter.