Re glider histories - there's Richard Cawsey. He generally knows more than appears on his website.
Slingsby page
Alternatively if you can work out at which club the glider was operated, an email appealing for info from the older members can work.
If you think it did any national or international competitions you can look through all the old copies of Sailplane and Gliding on the Lakes GC website.
The problem is that most people used a personal racing number which was transferrable from glider to glider rather than the fixed BGA number and trigraph so tracking can be difficult