Silly question but just to make sure, you have contacted the appropriate people in your previous company and pushed them to provide the reference explaining the seriousness of it all. I had to badger a previous foreign employer for some time before they coughed up.
I was told by the referencing agency that a tax declaration would suffice but this (being from a foreign country) would have been an even bigger pain to attain involving either trips back to the country or an appearance at an embassy.