"agileflower" replied after
Bob Hobden asked
Which way does the wall face?
Does it get much sun?
Is it a dry area?
What's the soil like, sandy, clay.....?
Roughly where in the UK are you?

That will teach me to write posts while chasing my toddler around!
Sorry--doh.
I am in north London. The wall faces E/SE. The soil, well-- clay soil
mixed with what seems like building rubble (!) It gets a reasonable
amount of late morning to early afternoon sun....

Having moved here from lush California, I can't quite get used to
gardening here. Before I could easily follow the rule 'stick it in the
ground and it will grow'

The small conifer in the middle should not cause a problem except they don't
stay small, even the so called dwarf ones get too large eventually. You
could move it somewhere else.
Your suggestion of Standard Roses (slightly over 3ft tall!) underplanted
sounds OK but I would incorporate some compost or well rotted manure first.
Roses like a rich soil.
Personally if I wanted that sort of effect I'd use the hardy Hibiscus and
keep them pruned well.
You could try a herbaceous border with lower growing plants and in that
position I would think some of the more tender plants could survive the
winter, Penstemons, Osteospermums etc. and plants like Asphodeline lutea and
Belamcanda.

I bet there are plants growing here that couldn't survive in Ca, it's just
different, that's all.

--
Regards
Bob Hobden
17mls W. of London.UK