Personally I wouldn't ever plant leylandii......if you want something similar look at Thuja
for more ideas go to H at the bottom of the page then scroll down to hedging and look at other members ideas.....hope it helps

It'll need to be drought tolerant to cope with all that established conifer root.

How big is the gap?
If you want easy maintenance then forget the leylandii - you'll be forever trying to keep them cut down. If you have a big enough gap then go for native deciduous hedging to counteract all the evergreens.

If you want conifers again, yes Thuja every time against Leylandii. For evergreen, quick-growing, then Prunus lusitanica (Portugal laurel) or Berberis darwinii, an evergreen shrub, orange-yellow flowers and berries.

Please find a photo showing the gap. I like the look of the Portugal laurel so I may go for that. What spacing should I gave between each plant? Thank you

Depends on how much you have to spend will determine the size of shrub to begin with and how long you are prepared to wait to fill the space. If they are about 3 feet high, then about 30" apart and if you can purchase larger shrubs, then the spacing can be wider. I would chop any of the roots from the leylandii which are likely to infringe on your new planting and regular watering of new plants is paramount for the first season.