I've just picked up a copy of invasion (at a very reasonable price of eleven pounds!) I just want to know if taking liberties and the guv'ner are separate stories? or is guv'ner a reprint that include liberties? If not, I can't find liberties at a reasonable price, would I be able to skip that novel and jump straight to the guv'ner? or is it essential to the plot? thanks.
Taking Liberties reprints the first three modern Invasion stories, The Guv'ner reprints the next three.
While it is a single, ongoing narrative, there are a a couple of deliberate changes of style and setup in the series which might mean you can read them in isolation. I can't remember exactly when those happen though.