@andymc I have a couple of simple apps on the play store. I get steady sales via simply upgrades, basically pay $0.99, £0.99, €0.99 etc for the full version (in your case more difficult screens). Maybe you should seriously consider do that. I personally hate popup ads in apps, if I like an app but I can't pay to remove the ads, I just uninstall the dam app, really annoys me lol
There's two idea for you to try and monetise more on your great success. BTW if I search for Space Invaders in the play store, your game appears 2nd in the search results over here in good old Blighty.