[arobwk]

Gardening client has bought some creeping phlox for soil-capped stone wall (for me to plant).  Potentially good choice for dry-ish wall top, but narrow flowering period in Spring I seem to think (and I guess she won't see any colour this year).  I looked up Rock Cress, but that seems to flower late Spring.  Mesembryanthemums could be an option perhaps, but any other suggestions please for something similar to add some Summer colour as well ?

[Steph Hen]

Corydalis lutea, yellow
Aubrieta
London pride.
This is from my old gardening ideas journal I made years ago and just redound yesterday and noticed it flicked open to this page!

[arobwk]

Clearly my timing was perfect @Steph Hen and I'm really liking the suggestions.  Thanks kindly

[Fleecewife]

Low thymes are flowering at the moment and look beautiful, with the added bonus of attracting bees etc.
London pride is a saxifrage and there are plenty of different types of saxifrage which flower through spring, starting with S.oppositifolia (purple) and carrying on with named varieties such as Boston Spa (yellow), pixie (deep pink) and some bright whites.
There are two types of cushion phlox - douglasii which is very small but perfectly formed (my favourite is Eva), then there is the larger variety which is probably the one your client has bought and is more impactful, making cushions up to about 3' across in time.
Campanulas have various forms including cushion types which are a beautiful blue and flower for quite a long time.  I have several varieties which have self seeded around the rocky walls of the waterfall in my pond - beautiful
There is a tiny plant called Fairy Foxglove (I've totally forgotten its proper name) which will self seed on the sides of a wall as well as the top - it doesn't have a long season but as part of a succession it's wonderful and so delicate, if it likes the situation (my last one has died!)

[doganjo]

Bear in mind what area you are in.  Plants flowering up here in Scotland have already gone over in many parts of England and wales.

[Fleecewife]

There are also dianthus (beautifully sweet smelling but might need a better soil pocket than some), helianthemum (rock rose) which comes in a variety of colours and lasts longer into summer, and of course various sedums and sempervivums, which provide deep reds and yellows in the leaves as well as flowers early.  There is also a tiny white-flowered low plant which starts flowering very early in the season and in my garden is still going strong.  Helpfully, I have no idea what it is as mine appeared from nowhere!
As well as how much soil there is available on the wall, alot depends on the rock type it's made of - acid, alkaline or neutral.

[PK]

Possibly too late  to suggest but Erigeron karvinskianus would do very well on a wall top. They have an informailty that suits that type of setting too. They flower all summer long. They are self seeding and you won't need to replant each year.

[arobwk]

Never heard of E. Karvinskianus BUT I shall obviously go check out shortly - thanks PK. 

Client likes roses so I'm also presently checking out "ground-cover" rose varieties that tend to spread well, but stay below 0.5m in height. 

[arobwk]

Well I did check out E. Karvinkianus:  I do know this one after-all & I would rename (if I could) as E. Cantstopitgettingeverywhereianus.  I might not follow your suggestion @PK .  Thanks though.