Probably shop it back and then dig it out. They are farily easy to remove ive found esp as the winters ruined every one i know of!

Is cutting it to ground level and making it start again not an option?
If not, then painting the leaves with a systemic weedkiller is a possible solution although this may take several applications

Many thanks for both your replies. I was hoping the winter would do the job for me Nicky. It's looking half dead but also quite green in places. I can't dig it out myself and my husband won't do it for me as he likes it. I was hoping for a sneaky weedkiller type solution so if you think this would work Andrew I'll have a go. Would I need to paint all the leaves and would I need a high strength product?

I would start off painting the leaves that are growing strongly and see how it reacts. I'm not sure if the product comes in various strengths - you'll need to ask at the Garden Centre

The best way to deal with an overgrown phormium, is to reduce the base, go round the base and cut the swords off at ground level, then cut down with decent spade or preferably a mathook, now that you have it to the dsired size rake the pebbles to the edge of the phormium, and its now looking good, take off any tatty swords[leaves], also if you do weedkill it then it still requires digging out.julien.

Thanks for that Julien, it's got me thinking that maybe it could stay. It's worth giving it a go although I planned to cover the dead stump with pebbles or larger stones rather than dig it out.
Thanks Andrew, I'll do that if I decide it has to go!