What about the Fenian parts of Ulster?
Them, too, forgot about NI. Also maybe some wealthy areas on the edge of other cities, like Sheffield Hallam. What is Plaid's attitude re: Europe?
Not sure, but I think it might actually have a better chance in Sheffield Central (and similar types of constituency) than in Hallam. I suspect it would struggle everywhere in England at the moment, though: the Greek crisis did not help the image of the Euro here, and as doktorb said the media are ridiculously anti-EU.