Thanks very much for all your welcome messages, and encouragement.
But there's no getting out of it easily!
With a bit of detective-work and searching through boxes of bits, I've found a few missing parts and it looks like the only part missing now is the bobbin case. I've thrown together a little montage to show the arrangement, -hopefully some wise guru will be able to tell me if anything else is missing. See my photo of the underside of bobbin area, and illustrations from the handbook (Thanks very much to for putting a downloadable manual online).
(Yes Leon it's a zig-zagger)
Cheers, Jools