Now that its all working, it appears to be quite a bit harder.... Closer inspection shows that the changes to the bullet collision have actually made it tougher! This is because coins can now be hit! So its pretty tough to shoot a line of aliens if they all drop coins, and means you really need to shoot it close to you, and then quickly pick up the coin so you can shoot the next one in line!
This is pretty hard, particually if your being hammered from all sides by turrets! I could change it now, but I think I'll wait and see what the feed back is. Of course, the temptation of gamers it to make it reasonably easy so they can finish it, so all comments will have to be taken with a pinch of salt, but lets see what you all say.