Wild flowers might be his best bet for the steep slope - I can't think of anything for the vertical part! - the lime loving ones are often lovely, lots of blue ones and most attract bees. If you type Wild flowers chalk into Google you'll see several suppliers of both seeds and plugs.

The Royal Horticultural Society has a list of lime lovers of all sizes apps.rhs.org.uk › Gardening › Advice‎ (I don't know why this hasn't appear as a link but if it doesn't work type in Shrubs Chalk and look at the RHS listing.) this will also suggest other plants for the foot of the cliff and the rest of the garden.