This looks (and plays) very much like Transversion on the Spectrum -
[EDIT] Sorry, the YouTube player her on GLB looks like it's broken - says it's an invalid link when put into the YouTube bb code, but it's not. So direct link only.

Your version is good

The only issue I have with your version is that it desperately lacks pacing - the enemies just move and fire too slowly to feel any real pressure through the first few levels. Also as far as I got, I didn't actually use the fire button. Is it actually required?