Dingle?
Have you checked out Harris Cyclery?
From there Sturmy Archer page:
seen on this page
"Special! NOS Cyclo Conversion Sprocket Sets! Quantities Very Limited
We have a very few New, Old Stock Cyclo 2- and 3-sprocket adaptors to convert Sturmey-Archer 3-speed hubs to either 6 or 9 speeds.
These work with 1/8" (wide) chain only. You must join the chain with a chain tool, not a master link. They work with Sturmey-Archer hubs only. Not suitbable for use with coaster-brake models.
Modern derailers are not wide enough to handle 1/8" chain without modification. You will need to add spacer washers to each side of the derailer pulleys to space the derailer cage for use with 1/8" chain.
Price: $49.95 for 2-sprocket type, 59.95 for 3-sprocket type."
Good luck & have fun!