ENA5 can be a tricky paper as you’re expected to have a decent knowledge of language change over time & attitudes towards change, as well as a sound grasp of more advanced elements of grammar. You can revise the change aspects using a range of online resources, but the grammar bit needs plenty of practice. There are some resources you can have a look at to help you test your own knowledge of grammar, and then you can try to apply this knowledge to some older texts. Try the Internet Grammar of English and English in Use sites and then take a look at the History of Prose Style and British Library online sites for examples of older texts.
I’d suggest looking at really short extracts of a range of forms and genres and see what you could usefully say about particular lexical and grammatical features in each text. So if you’re looking at one of the selection of recipes available on the British Library site, you could think about pronoun use, modal auxiliaries, sentence function and uses of subordinate clauses of condition in the context of what the recipe’s main purpose is.
Alternatively, with a piece of philosophical or political writing, you might get mileage out of looking for particular noun types, semantic fields and sentence types: you might find that there are more complex sentences featuring relative or dependent clauses, because that type of text requires more complex logical relationships as part of its subject matter.
For the contemporary language change essay question, it’s important you have a range of examples of new words that have come into the language over the past 50 years as well as the linguistic terms used to label the processes that have created them (clipping, compounding, blending etc.). It’s also useful to look at semantic change and have the same set of terms at your disposal to describe changes in meaning (pejoration, amelioration, semantic reclamation, broadening etc.). One of the best places I've come across for discussion of new words is the MacMillan English Dictionary site.
Try to get a sense of the reasons for change too: new technology and the influence of the media are two such factors but there are many wider ones to do with social and political change (gender, sexuality and race issues and the language associated with them) and education.
With attitudes towards language change, I think it’s important to know your history and realise that people have always moaned about how language changes, while others have embraced the changes as part of a natural organic process: progress, if you like!
So, look at the background to things like Samuel Johnson’s Dictionary of 1755 and Jonathan Swift’s calls to introduce an academy for language. The best places I’ve found to mug up on these have been Melvyn Bragg’s Adventure of English and the BBC series, the Routes of English .
If you can get hold of it, David Crystal's new book How Language Works has a good section in it about change and the whole prescriptivist/descriptivist debate called "How Not to Protect Languages".