If a solicitor sends a letter to someone to be passed onto a third party and the third never receives it but it was sent on, how do we find out which solictor sent it bearing in mind this would be 15-20 years ago. We believe we know who instructed the solicitor and believe the solicitors are in Newcastle or Durham. We need advice on how to start tracing where the letter came from.
Can you help

The fact this happened so long ago kind of makes things EXTREMELY difficult.

Most companies only keep correspondence for, up to, SEVEN years.
Also, as you are in the UK, Data Protection and retention laws come into play.

Do you know which solicitors it originally came from ?

After 7 years do they not archive documents and no we don't the solicitors that sent the letter.

Really depends on the subject matter.