I purchased a copy of the CD - St Mary's Lydney Memorial Transcriptions back in 2010 which contained a churchyard plan. It showed all of the headstones, flat ledger tombstones & box table-top old tombstones - all numbered & arranged in blocks around the church. This CD was the work of a lot of people in the Forest of Dean branch of the Glos FHS.

I looked at the website of the Glos FHS (as Jeff suggested) & found that the CD I purchased then seems to be not available anymore. However, there is a CD entitled: St Mary Lydney - Monumental Inscriptions in stock priced at £5 on their website shop. It might also contain a churchyard map?
However, if your ancestors did not have a stone to mark their resting place, then their exact burial plot would of course not be shown on this map.

As the churchyard of St Mary's is so huge, I found the map & reference numbers of the graves invaluable in locating the tombstones of my Benfield & James ancestors.

I cannot find any mention of a plan within this forum's old posts, or the rest of the site itself.

Have you tried asking the Church ?
http://www.lydneyparish.org.uk/contacts.php

This site doesn't have a plan, but if it included the burials you're interested in, then the photos should help.
https://www.findagrave.com/cemetery/2429303/memorial-search?page=1#sr-197304026

I could be wrong, but thusfar I cannot find any mention of Lydney in the various cds sold by Glos FHS.
eg http://gfhs.org.uk/products-page/cddvd/monumental-inscription-2003-2/

??