Filling the gaps

Thalictrum delavayii, Verbena bonariensis and Penstemon are good, reliable gap fillers. How about trying some ornametal grasses, kept in tubs (so easily moved if necessary?

Thanks for the ideas.Although our garden is South facing this border is partly shade by some trees so I wasn't sure if Salvias would thrive.I have tried a terracotta achillea before but after a very wet winter it never took off.Maybe I will try again.
In the meantime I remembered a lovely nursery a bit further from home and went today.They were really helpful and I came home with purple lobelias,dark blue veronicas and large red persicarias which they advised were self supporting and fairly bomb proof.But ideas always welcome as still a bit more room !

What about something like this?

www.woolmans.com/Chrysanthemums/Chrysanthemum-Mrs-Jessie-Cooper.html#.XYzfnS2ZPwc

I'm a big fan of achillea for height in a border - long flowering season. Bees and butterflies love them. The colours might not suit you though: sharp yellow, golden yellow, terracotta.

I love those hot lips salvias Shysal, they're quite a talking point!

https://www.gransnet.com/uploads/talk/201909/medium-999630-salvias.png https://www.gransnet.com/uploads/talk/201909/medium-177450-hot-lips.png

What about some Salvias as discussed in the 'What is this?' thread. They normally grow to about 3 ft. and have a long flowering season.

Red hot pokers?