By Andy Bruce LONDON (Reuters) -The Bank of England sought again to stem a sharp sell-off in Britain’s 2.1 trillion pound ($2.3 trillion) government bond markets on Tuesday, expanding its emergency buying to inflation-linked debt. Citing a “material risk” to financial stability after pension firms were hit by the turmoil, the BoE split its programme to buy up to 10 billion pounds of British gilts each day to include up to 5 billion pounds of index-linked bonds. The expansion of the purchase programme was the BoE’s fifth attempt to quell market turmoil in just over two weeks, including verbal i…