A mate of mine had the Spectrum 16k (or was it 32k) with the expansion pack (about the same size as the spectrum) to make it a 48k. I just got the 48k version. Jetpac was a great game.
Started with zx80, then went to 16k speccie, then 48k, then went downhill with the 128k. Jetpac was ok, but was blown away later by JSW, Lords of Midnight, ZZoom, and HOTT as earlier, yes i'm an 80's cg anorak before anyone says.