You could slide some Carex 'Ice Dance' in there; it runs at the root a bit and I would imagine would pop up and fill any gaps it can find among the Phlomis (and it's evergreen and always looks pretty tidy).
If this bed is completely contained you could pop Euphorbia cyparissus in and see how it fared in a 'battle of the thugs'.
"What is hateful to you, do not do to your neighbour".
Not sure I can cope with more than one thug at a time! It's not completely contained - it'll open out to a squareish bed at one end. I do like euphorbia...
'If you have a garden and a library, you have everything you need.'
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If this bed is completely contained you could pop Euphorbia cyparissus in and see how it fared in a 'battle of the thugs'.