There are various possibilities for why the bottom part is not green.

I suspect the most likely one is that you have trimmed it too hard on the sides in the past. Leylandi and most other conifers do not regrow if you trim off all the green leaves into the old wood.

Other possibilities include disease or insect attack but these are unlikely as the tops where you haven't trimmed too hard are fine .

Unfortunately there is nothing you can do to make the lower parts of the trees regrow

I think that it is the disease that is affecting Leyandii trees and it just hasn't risen all the way up yet. Nothing you can do about it I am afraid, Michael, except remove the trees and plant something different.

Thank you very much for your replies. A couple of seasons ago I did cut them back quite hard and this is probably what has caused the problem. It may well have also been compounded by disease as the privet type bush in the middle is showing an area of browning lower down but is also quite badly infested with white fly. All in all a bit of a mess really!
Can I ask what you would recommend I plant in their place and what time of year would be best to remove and replant. I need something that will form a pretty substantial hedge as there is a main road on the other side.

It does sound a bit of a mess, Michael, so I would suggest removing them as soon as possible. Then you can dig some decent compost into the ground and let it settle before replanting. You presumably want something that is evergreen and a bit more colourful than leylandii and privet. Lilac or buddleja could be used or a varigated holly, maybe. Go and see what your local tree nursery has to offer.

You could choose several different evergreen flowering shrubs.

Here are some suggestions

Some of the evergreen cotoneasters make excellent hedges. I planted one in a customers garden a few years ago. It has lovely white flowers in spring followed by red berries.It will definitely grow to 6 feet tall but needs staking for the first few years until it becomes self supporting

Here's a close up photo of my customers hedge in flower

http://www.flickr.com/photos/31559373@N00/3554081853/

Viburnum tinus

http://www.flickr.com/photos/31559373@N00/3379402503/

Escallonia

http://www.flickr.com/photos/31559373@N00/553286521/

Photinia

http://www.flickr.com/photos/31559373@N00/3507421133/

Choisya

http://www.flickr.com/photos/31559373@N00/3508225698/

Variegated holly

http://www.flickr.com/photos/31559373@N00/4760898250/

Ceanothus

http://www.flickr.com/photos/31559373@N00/3526268098/

Hypericum

http://www.flickr.com/photos/31559373@N00/5912171528/