Thanks for all the great advice guys
It appears the Cyber Gun Mini Uzi is basically a bit of a madam.
It also appears she likes to be lubricated to the point where she's practically dripping (ahem)
Having totally dismantled her (thanks for the vid moz) cleaned and lubricated again this morning, I've now put 3 mags through her with not a single jam.
I'd imagine the jams will be back at some point but at least we now know how to deal with them.
Incidentally, I'm getting 3 mags worth of shooting from just one CO2 which is better than expected.
Now all I have to do is wait 7 days for Santa to bring me the CP88 6" Nickel that I happen to know is hiding under the spare bed.
RESULT