I'm not sure what you mean. It is dated and it was never a reference book, it is popular history.
It may be a great read, and thought provoking, and for these and other qualities I expect it is still worth recommending, but it appears that it is not always so reliable a reference, and compared to more modern books, has some shocking omissions. For example, (I'll note that I am quoting from 1995 edition, although this does not seem like the sort of thing that would change in an update, since this the viewpoint that Roberts offers is part of what makes the book distinctive) page 569
''The year 1688 gave England a Dutch King, Queen Mary's husband, William III, to whom the major importance of the 'Glorious Revolution' of that year was that England could be mobilized against France, now threatening the independance of the United Provinces (='Holland')''
Read that very carefully.(Also, see PS) Now, another popular historian, Andrew Marr, heads his section on the same topic with the pithy title 'Britain Invaded', (324) which, although it was perhaps more like a coup and was later made to appear 'natural', a continuance of native tradition, is not an unfair title, as there was
''... a huge invading force, twice as big as the Spanish Armada. The Dutch fleet of fifty-three warships and about four hundred supply ships had outwitted the Royal Navy, sailing first towards the east coast of England and then using a change in the wind - 'the Protestant wind', people said - to sail west, landing at Torbay in Devon.''
''William had taken a massive military and personal gamble because the Dutch Protestants were desperate [later, tensions with France are discussed, ie the reasons why they were desperate are given]. Had James not panicked and fled, or had the wind veered a little, William might have lost everything. Instead, with his wife, Princess Anne's older sister and confident Mary, he became part of the only joint monarchy in modern British history.''
It is quite shocking to miss all this out, a shame for a modern reader to be one of those ''Even quite well educated people [who] believe that England has not been invaded since the Normans arrived in 1066.'' (Marr). The problem is, you wouldn't read Roberts and then think 'I'd like to learn more about that invasion', since unless you are very good at reading between the lines, you would not know that there was one. I'm guessing you were referring to something else when you called it 'gob-smacking', but certain omissions could be called 'gob-smacking'.
I think many people would prefer to not have the omissions and 'spin' of Roberts if they were relying on only one book, although this would probably make a good companion to other world history books.
PS and tell me if there is anything I missed, I haven't read much of Dr. Roberts' book yet, but further consultation of the index only yielded ''A turning point [in the franco-dutch conflict] came in 1668 when William of Orange, the Stadtholder of Holland, took his wife Mary Stuart to England to replace her father on the English throne.'' Still gives the impression that they were invited, no mention of a force ''twice as large as the Spanish Armada''.
And so it is quite conclusively wrong, p569, saying that ''ENGLAND [...]put James I's daughter and her consort on [the throne]'', as if it were a free choice, not the consequence of being invaded. If anything is gob-smacking, then there it is.
PPS And I've read the relevant passages in The Oxford Illustrated History of Britain (Oxford Illustrated Histories) first edition 1985, my edition 1991, and from reading that alone you would see how outdated DR. Roberts' analysis is, notably (in response to Roberts) saying how the Tories would have preferred it if the Dutchman just put Mary on the throne, but he demanded to be joint ruler and England had to do as he said.