DISPUTED LaND by TIM PEaRS Heinemann ?12.99 ebook ?13.56 pp210  Tim Pears's seventh novel presents a comedy of middle-class manners through the lens of science fiction. The year is 2058, and Britain is short of both food and fuel. To explain how this came about, the narrator, Theo, looks back to a family Christmas with his grandparents five decades earlier, when he was 13. His story is dramatic - featuring a brawl and three fatal shootings - but the interest lies less in its uniqueness than in its potential to sum up what Pears clearly believes is our present-day decadence.  It begins with Theo's parents driving to Shropshire. Theo switches off his MP3 player to listen to them discuss their loft conversion and "the historically proven impossibility of an occupying army imposing peace in afghanistan". a teasing tone here lays down a formula for the portrayal of some predictably odious relatives from Hampstead: Uncle Jonny, a wouldbe property developer whose mobile phone keeps bringing bad news about cash flow; his argentine trophy wife Lorna; and their obnoxious twins Baz and Xan, who are baffled by their grandad's use of a barograph to forecast the weather (they have an app for that).  Environmental themes are aired amid this parade of stereotypes by Theo's grandmother, who rants about overpopulation and climate change. Because these dinnertable tirades turn out to be the symptom of a serious illness, rather than the result of rational reflection, nobody takes much notice; except for Theo, who, in the future, has to resort to burning books to keep warm.  The solemnity with which