Thanks Chris. It's what I'd suspected; it seems the UK market for 3/32" cogs has shrunk to a point of very limited choice.
It could be worse. I doubt that either Surly or Andel cogs are normally eccentric, since that should be a function of production machining procedures (give or take a bit of quality control), as opposed to the old Cyclo cogs, whose teeth, I suspect, were stamped, with inaccurate reference to the hub threads.