FTSE 100 heads for fourth session of gains

(Reuters) - London's FTSE 100 held firm at a near one-month high on Thursday as gains in AstraZeneca after a 2019 target upgrade countered losses in Royal Bank of Scotland following a weak third-quarter update, while midcaps outperformed as sterling rose.The blue-chip index inched up by 0.1% at 0708 GMT, while the FTSE 250 index gained 0.2%.AstraZeneca added 2.3% after the pharmaceutical giant raised its annual product sales forecast for the second time this year, thanks in part to strong..