If you can obtain evidence showing the common law relationship ended back in 2005 or 2006 you should be just fine. Letters from friends/family would work. If you filed your tax returns as "single" after that time, you should also use that to demonstrate your change in status. Leases for apartments would also potentially show that (e.g., something in your own name). This would be particularly good if you can show before and after ("2004 lease in both names... 2006 lease in your name alone" for example.)
You should also order a copy of your GCMS case notes now, because the "notes" section at the end will probably give you insight into what the officer really is concerned about.
