You will find at the base these large phormiums are really a collection of smaller plants stuck together, just separate the sections to reduce ( can be tough!) but if you get some roots with them these will make more plants for free.Just cutting them back will not look good.

Hi, welcome to GoY, I agree with Dgw, you may want to arm yourself with a good pruning saw, Derek.

Just to add that this is best done when the worst of the frosts (if we get any) are over. Late March or April. The old roots in the centre are usually useless and the fresh roots around the outside are the ones to keep. Good luck with the new plants.

Alternatively, dig them up, cut the rootballs in half and replant one or both the sections. Can be a bit of a job if you've not done it for some years... I tend to do mine every 2 to 3 years, and bin the portions I don't want if I can't find someone who wants them.