First off, welcome to the asylum, Dave.
It's good to see that Rich has responded. He's the resident Lightning Clubman guru.
I'll second the point about the dispatch records. According to Google Maps (so it must be true), Stafford is only about an hour's ride from the VMCC library
in Burton on Trent. That's apparently available to non-members, and they have almost the full set of BSA dispatch records on microfiche. The BSA Club also has a copy of the records, but I'm not sure what the availability is like.
Both the BSA Club and VMCC will provide a dating certificate for a fee as well, if you can't manage to check the "books" yourself.
The story about it having been a factory hack is intriguing. That should show up in the dispatch records, though they do require a certain amount of effort to interpret.