I found a case of Scottish beer / bitter that I had forgotten about, it is out of date by 7 months, which normally would not stop me but it tastes watery. My question is can I re-ferment it or add something to resurrect it?
I seriously doubt it. What happens over time is that chemicals giving the taste decays, and you cannot magically make them appear. You could perhaps add something to give it taste, but then, is it still beer?
Refermenting (could be done by adding sugar) will give carbon dioxide and alcohol, but not more taste.
Could pour it into something like this and infuse it with more hops or adjuncts.
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If it's not a beer with a high alcohol percentage (say 7%+) I'd just throw it out, life's too short to drink stale beer.
Otherwise you can test several bottles. You might get lucky with a few, not every bottle in the same case ages at the same rate, certainly if it's one with fermentation on the bottle.
tip: If you hear a hiss when opening the bottle, it's safe to taste. I never trust a bottle that's silent when opening as it almost guarantees that it's gone bad.