Thanks Alan, Wigas and Tommy for your in-depth responses. Really appreciate the insight!
Alan - no notifications on Webmaster Tools. Feel like I'm between a rock and hard place with the spammy links. Google does say if you upload another file it will completely override the first disavow file, so perhaps I should consider uploading a new file with only a couple of bad links, thereby negating the original file with over 800 spammy links.
Wigas - Crawled through Moz showing over 100 critical fixes including configuring attachments as no follow/no index with redirect to main site. Need to work on improving the backlinking!
Tommy - Disavow is just a mystery. Bottom line probably is if you don't have any warning notice in Webmaster Tools, don't disavow, just improve. Will revisit title-tags and image weights for sure to improve response time.
Certainly have a big mess to clean up!
Thanks again!