Cheers Tom

I called an aquatic supplier I've delt with over the years in Scotland and he described (at enourmous length) how they do it..

They use a CO2 proof T peice and 2 CO2 proof plastic valves - he said they use a single CO2 cylinder per 6 tanks via this method and some of the distances from cylinder to tank are over 100'. He admitted it is fiddly getting the balance right, but he assured me that once set they don't tend to drift...........

The cost is minimal so I ordered it and am keeping my fingers crossed - proof of the pudding etc....
I'll post an update when it's working - or not

If it doesn't work, then I've found some other US-based suppliers who will ship 2-port manifold with needle valves to the UK for under $50

Best wishes
Pete