Thanks everyone for the various suggestions. We already have various grasses in the main part of the front gravel garden, including Panicum and Acanthus (completely under control so far!), and so maybe I'll stick to the tall grasses theme interspersed with flowering plants. Helianthus "Lemon Queen" looks like a good addition as well as some ground cover plants.
I know you’ve said no shrubs but a drain at 0.75m is quite deep for a number of evergreen plants that would hide your fence through the winter - if that’s the purpose of the tall plants?
Anyway, my two pennyworth of suggestions which might have some winter interest if you don’t cut them back:
Thalictrum Flavum Achillea Lychnis Coronaria Verbena Bonariensis Eupatorium Rugosum Chocolate Cirsium Rivulare (this might die back completely over winter - I’ve not got one in my current garden) Hellebores (small plants but will provide all year round interest if you choose one with marbled leaves with a good tolerance for sunlight)
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Anyway, my two pennyworth of suggestions which might have some winter interest if you don’t cut them back:
Thalictrum Flavum
Achillea
Lychnis Coronaria
Verbena Bonariensis
Eupatorium Rugosum Chocolate
Cirsium Rivulare (this might die back completely over winter - I’ve not got one in my current garden)
Hellebores (small plants but will provide all year round interest if you choose one with marbled leaves with a good tolerance for sunlight)