Bring a medium pot of boiling water to the boil over medium heat. Season the water with salt, then add the orzo and stock cube, and cook until just soft. Drain, reserving the cooking liquid.
In a pan, gently melt the butter and fry the leeks until translucent. Keep heat low, as you don't want them to brown. Add the sweetcorn, and 1 to 2 tablespoons of the reserved orzo cooking liquid. Bring to a simmer and cook until it thickens.
Add the orzo and toss to combine. Season with salt and pepper, to taste.