If you mean the plant with the brownish leaves growing in the centre, that's not a cordyline, its a Phormium. They don't develop a trunk like cordyline, so reducing their size requires different treatment. You need to dig it up, cut the root ball in half, then replant one section. This will reduce the width, obviously, but won't reduce the height - its now at its optimum height and shouldn't get any taller anyway.

Well thank you for that the gardener said it was a cordaline it just takes up the garden it don't show the real size on that picture thanks again.

If the rootball is large (not easy to dig out, but certainly do-able) then you can cut it into more sections and just replant one of those - that way, it'll take longer before you need to do it again.

Thanks but it's only been in eighteen months and has got too big so I am going to replace it with a Japanese dwarf acer

Hi when I replace my phormium will I be able to but my Japanese Acer straight in or will I have to wait a while.

Provided you remove the roots of the phormium properly, dig over the spot and add some composted materials (garden compost, soil conditioning compost, that kind of thing), let it settle for a few days (up to a week), you can then plant the acer.

Thank you Bamboo

I should add that Acers don't like limey soil - neutral to acid is best. If your area has pieris and rhododendron/azalea growing happily in the ground, then the ph will be fine.

That's good because my soil is acid I do have pieris in my garden cheers for that.