@pfaffman : Thanks for having a look.
No, all I did was add a series of variable definitions that pertain to the site name and the use of tags
db_shared_buffers is “4096MB”
unicorn_workers is 8
db_work_mem is commented out
I have one main 40G partition (14GB free), 512MB swap, and an 8GB partition for backups (not mounted).
It appears I have overcome the problem. Initially I tried reducing buffers to 2GB and workers to 4 but got the same error. I then reduced the buffers down to 1GB whereupon rebuild succeeded and the forum is now back up.
Thank you all!!