Another (belated) quick update:
I'm still here, time has been very limited of late but I'm still chipping away at it.
Sprite movement is working correctly now
Collision detection is in
All 20 levels can be played through now with minor bugs to be fixed
still got some enemies to code in for the later levels (tentacle, raft, crushing walls)
level complete routine done
still fits on a single floppy, and I haven't compressed any files yet!
known bugs/problems to fix:
Graphical glitches before screen change, intro to titlepage to game and back etc
music in outro is playing badly, may need another playroutine despite being in the same module as the rest of the music
laser goes through walls and hits baddies at the moment
explosion sprite's colours look off
explosions of walls and baddies continue to the next screen and reappear on previous
lava is a little high up, need to lower it or game will be impossible!
seems to run sluggish without fast ram installed... (not sure why yet)
But I'm still getting there!