Depends what you want! You can hack them fairly hard and they bounce back or use shears and just trim to shape.
I've always heard that cutting back too hard means they won't grow (from old wood) ... the plants are relatively young but the stems are sort of woody ...
I find that when you cut them hard you will find new shoots on the old wood and provided you leave them they will regrow. I inherited one and it was huge, last Spring I hacked it back and it has rejuvinated nicely.
I can see new growth down in the centre of some of the plants in your pictures. You can cut back to that, give them a good drink and maybe a sprinkle of blood fish and bone meal or other general fertiliser, and they'll grow away.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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