There's a book called "Working Effectively with Legacy Code" by Michael C. Feathers which I heartily recommend.

I got it a while back when working on a similar sounding codebase and it was very useful. It's got chapter headings like "My Application Has No Structure" and "I'm Changing the Same Code All Over the Place".

If your company has a books budget get them to buy it, but I'd personally drop the £35 it costs on amazon given how useful this has been to me over the years.