Oh man, history extension was awesome! Really changes your perspective on what history actually is.
In terms of difficulty, it may be hard to grasp at the start because you're no longer just studying history in general, you're also considering how history is constructed, different intepretations by historians, as well as the historian's own context influencing their response.
It also depends if you have good linkage in terms of the two modules. For the second section i did Appeasement (between WW1-WW2), which linked well with what i was doing in Modern History. (To some extent, i think it boosted my marks a little too
)
Ah the major work, it's basically a research assignment but in the context of what you learnt from the first module- historiography. You're free to choose any topic (subject to your teacher agreeing it is a valid topic), you research the context, analyse various historians' intepretations of that event, comparing and assessing their pros and cons, then coming to your own conclusion. its great you can pursue any topic, and if you're a true enthusiast you'll enjoy the freedom of researching and- for once, coming to your own conclusions rather than blankly quoting others. But yeah, a lot of work involved in researching, keeping the diary up to date, and knuckling out a good research report.