Don't grow ivy or other climbers- they would grow up and engulf more of the leylandii and kill off more branches. You can certainly replace it with yew but you'll need to improve the soil when you make the changeover. Difficult to suggest ways of concealing the dead bits as you have taken out shrubs that were in front of it so presumably don't want more there.

Thanks for that advice. Can't replace the tree as it is about 8 ft high and is part of a hedge between us and the neighbours. Their side gets all the sun and looks good!

That's the trouble with conifers, going dead at the bottom. I've got a Viburnum tinus covering our worst gap, but it does depend how much room you've got.