Why go forensic though? The man had a paving slab thrown on his head. He got a fractured skull and brain damage - however, he was not murdered. Given that there were several witness accounts to the incident, and probably very little that forensic examination could conclude, did it really need to go forensic?
The issue here is the fact that vital evidence was not considered, or rejected, and that there the conviction has become increasingly unstable due to this all coming to light and an evidently stronger case against Graham Sankey being built. So, should Shields be pardoned? No. Because that suggests while the British courts accept the guilt, they don't accept the sentence. Should there be a retrial? Yes. The evidence supports it and casts enough doubt in my opinion. Not to mention that seeing as Martin Georgiev will be brain damaged for life by these hooligans, he deserves justice as much as Michael Shields does.