Thank you all for the advice. I accept now that it is a phormium. I recently bought it from a plant sale and lost the tag. It probably was not getting the light it needed. I have repotted it into the clay pot although I will watch it doesn't get pot-bound. I will eventually find a place for it in the garden. I would like to know how big it will get though before planting it.
Here in Suffolk, we are having a very dry spell and it is hot this weekend. The plant isn't dry though. I have been having to use my hose on my garden this weekend so save my plants from drying up altogether.