Phormium tenax, the New Zealand Flax. Just let it get on with it....seems to be doing fine. No trimming or pruning required although if it gets too big, they might need digging up and splitting every 10 or so years.

Hi, welcome to GoY, the plant is Phormium tenax, or new zealand flax, the leaves can grow to about 10 ft, and the flower panicles up to about 13 ft, spread is about 6 ft, the leaves are very tough, you arebest to use a saw, or an old bread knife, to cut out the leaves right at the bottom, or as low as you can get, this year has been a good year for them to flower, but they tend to flower in good summers, they are not completely hardy, but yours should be ok, as it apears to be in a sheltered position, they like full sun, Derek.