Finally coded up the nebula effect. Came together very easily. Just rendered some of the late frames of an explosion (with some colour and settings tweaked) and drawing 100 scaled up sprites, with varying sizes, velocities, size and transparency. You get 100 lovely layers of parallax prettiness.
Also added a variant of enemy that comes down from the top then spins off at an angle sideways. Pesky critters are hard to shoot!
A couple of new screens: