I agree that it is almost certainly safe - as you point out, how on earth could pathogenic bacteria have got in and multiplied especially in the fridge ? Food safety is so much commensense. Having said that...... a pack of mackerel ? Maybe �1. For ultimate peace of mind don't eat it, but I'll bet it's perfectly safe. I have eaten runner beans that have been in my freezer for 5 years.
The logic is surely, if it was safe when it was vacuum-packed, then it must be safe now. At which point does it become 'dangerous' ? 3 months, a year ? And if so WHY ?