Good news everyone - I have a working board
The bad news is that neither of the OK boards are working yet
Because it shares all of the components from the OK apart from one transistor, components which I have, I built the Electric Brown and connected it up to the pots and jacks etc. from the OK board (still without the switch) and it worked first time
So with that I've managed to at least narrow the problem down to something on the OK board, either layout or component, which is good information even if I haven't tracked down the precise issue yet.