Boys and girls, thankyou all so much for the info. The disraeli site has been a superb source of info, I've probably spent three hours on there so far reading all about the good, bad and ugly of derailleur gears from the 40s and 50s. Superb resource, how lucky we are to have sites like that, manned by 'anoraks' to aide our research!
I was particularly interested in the personal experiences of gearing systems posted on this thread, there's nothing quite like the memories of cyclists who were there, did it, and have the tee shirt
I've decided that my aim for the Bertrand shall be a four speed Benelux, and maybe even with a double-clanger should I ever ride it outside of Norfolk and encounter something called a "hill" (whatever that is!)
For the interim while I track down the Benelux bits I need, I have obtained a Resilion, all sparkly and unused, and thankfully complete with a cable and shifter. A three-speed 1/8" block from the Old Bicycle Company, and my fabulous LBS who still had half a chainstay clamp in stock! I'll have the other half made.
The bike will slowly come together over the next few months, and I'll be sure to post pics to check I've put together something suitably period looking.
Thanks for all your help, memories and pointers, I'm really grateful