Yep that did it thanks
My camera origin was actually marked as center so I came up with the crude code as follows which works
Vector3 cameraPosition = camera.transform.position;
Rect extents = camera.ScreenExtents;
float screenX = Random.Range(cameraPosition.x - (extents.width * 0.5f), cameraPosition.x + (extents.width * 0.5f));
blockPrefab.Spawn(new Vector3(screenX, 26.0f, 0.0f), transform.rotation);