I am a bit of a dufus, obviously, but can someone tell me the keyboard key to fire the missile launcher on your ship?

Not all ships (seemingly) come equipped with one. I've actually not found any ship that has it, other than the starting one, when I made a new save.

I can only presume that the blueprint has to be bought or unlocked...

It's the regular left mouse fire key to launch them. You alternate/scroll through the weapons using the "switch weapon" key. I think it's "X" by default.

I didn't find them all that useful on the starting ship but there are probably various homing/speed/power blueprint upgrades out there in the universe.

Fire with LMB, use the "G" key to cycle ship weapons (same thing as on foot to cycle between tool modes)

You switch weapons with G, be it on ship or on foot.
With R you reload if necessary.

Thanks guys. Told you I was a dufus!

I feel the missile launcher should be on more ships, as with the laser having about 90 damage and the missile launcher having 3000, if you can hit the enemy with a rocket it's practically game over for the battle!