I admit that plum porter isn’t as good as it once was, seems to be inevitable when they ramp up production, same issue seems to affect a lot of breweries. , I.e. Hobgoblin while still very pleasant I would say is not as good as it once was.
Plum Porter is in my view superior on cask the pub near where I work in Oxford is part owned by Titanic, so it is a regular beer, and easily one of my first choices their, but even in a bottle it is pretty good for a semi widely available commercial beer.
If you see it it is worth getting the Titanic reserve which is a 6.5% seasonal version they do, also if you can get it they do a Cherry version which if you can get on cask is great.