Many thanks for your patience. First of all this is not discrimination because alcohol dependency is not a disability and it is specifically excluded from such protection. Therefore, you cannot claim for discrimination here.
However, it would be potential unfair dismissal based on several factors. First of all, if your contract was amended without your consent and you were not informed that the relevant period was increased to 12 months, the the employer used that change to argue that you had breached your contract, it would clearly be unfair of them to do.
When considering gross misconduct, the employer should have also taken into account the effect of you having had a drink, on your work. For example, were you actually alleged to have been under the influence of alcohol in work? Did they dismiss because you had simply had as drink, despite it having no effect on your work. It would be difficult for them to justify it warranted dismissal, if you had drunk outside if work and were not drunk at work and it had no effect on your work.
They should have also considered your length of service and disciplinary record, which respectively were very long and clean.
There have been instances in the past when even those who are alleged to have been drunk in work, were deemed to have been unfairly dismissed:
In that case you will see the dismissal was unfair because the decision fell outside of the reasonable responses expected of the employer and I think there is a good argument that this was the case here too.
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