Thank you to everyone who has helped identify this as L. rosthornii. I am now convinced that the seed was obtained as L,henryi and my dating it as twenty years ago is almost certainly an overestimate - maybe fifteen to seventeen would be more accurate.
I am determined to have another go at this species. Unfortunately, even if the plant survives at Kerrachar we won't ever be there at the correct season to harvest seeds so I'll need to seek out another source or buy a mture bulb.