We have a Leylandi hedge that has large gaps at the bottom between the lawn and the first few branches. can you reccomend a plant that will fill the gaps?

Unlikely that anything will grow as the Leylandii will take up all the water and nourishment

There are several plants that are perfectly happy in shady, dry, poor soil.
Ivy will grow there, I know because I am struggling to remove an ivy from my leylandii hedge.
Lamium comes in both silver and gold variegation.
Gaultheria procumbens. The Checkerberry is a prostrate evergreen shrub 10 - 15cm tall, it spreads by perhaps 10 - 20cm per year and so, if allowed, will eventually take up quite a bit of space. It may take its time to become established but it succeeds in dry soils and is quite drought-tolerant. A very cold-hardy plant.

Some ornamental grasses do well, as they don't need much nutrient and are great in dry areas. Bamboo is another plant for this.