Eventually I went for the paper stretcher. It seemed to be the better of all the solutions. Although the paper dries dead flat after stretching it doesn't stay flat whilst painting on it.
Once the paper was applied to the stretcher and dried over night it pulled really taught and flat. Unfortunately once a wash was applied it absorbed the water and began to cockle again.
I've tried it with both 300g and 190g paper. Althought the 300g tends to hold its shape better it still cockles.
Solution! Do smaller paintings or buy thicker paper. It's the only way to solve the cockling issue.
Incidentally, paper stretchers aren't cheap.