It's a Phormium, either the variety Cream Delight or Yellow Wave. It won't get taller than it is, just wider - it can't be reduced in size easily, I'm afraid you have to dig it up, cut it in half and then replant one of the halves. A sharp bread knife is the best implement to cut it with...

Thank you so much. It looks like I will have to get digging!
I'm half in mind to just move it to be honest. It's grown double the size in the last two years alone.

Try to get it out without damaging too many roots - they're quite fibrous and go down a least a spade's depth and will have spread out round the visible outer foliage too. It tends to grow in 'fans' so even if you lose some, some will survive.