The solution for me was an (earthed) laptop charger that I happend to have - it was rated 19v and 3.4A and had a 3 pin input to the transformer block, so I guessed it was earthed . Luckily it had (i) enough ampage, (ii) the same polarity and the same fitting as the power input on the BG-75.
With the earthing problem now solved, I think the Double 4 BG-75 is a wonderful little combo, highly portable, and with a more vintage sound than other PJB stuff . Its amazingly loud for its tiny size. I am using it in an acoustic pub session usually coimprising 5+ guitars, piano accordian, a trombone, bouzoukis, mandolins, 2 bodrhan players and other punters talking.
Alan, at Synergy the distributors, has been incredibly helpful and supportive throughout.
I'm now rather chuffed with this little amp, and I understand there is a possibility of running it off a battery in the future.
I await developments.