Hi Ross, thank you for the reply.
I'll go through and try that. I had already optimized the entire table in one go, which came back with this yellow warning (the same as when trying to create a new user).
http://i473.photobucket.com/albums/rr95/richardpetersphotography/DB_zps8bf22ce5.jpg
Also, you can see that wp_commentsmeta is where all the size is being eaten up. Originally it had 6000 or so records. I deleted them but now despite saying ZERO records, it is remains 76mb in size even after optimizing.
I can't download any plugins right now as I have lost admin rights - which was why I was trying to create a new user in the DB, when I discovered the bigger problem :(