Problem starting Fiesel 1.8D ( non return valve size )?

I am having problem starting my 1999 Fiesta Diesel.
So far I have renewed the glow plugs , no luck so far but I have seen on a previous thread from last year about fitting a non return valve if so where abouts would be best also what diameter one would I need. Also any other thoughts please. The Van has done 44,000 miles only.
Many thanks