* FTSE 100 up 0.8 percent

* Miners gain on Chinese data

* Banks up on improved risk appetite

* Sainsbury and International Power rise on firm results

By Simon Falush

LONDON, Nov 11 (Reuters) - Britain's top share index gained 0.8 percent early on Wednesday, as optimism on the prospects for demand was lifted by data showing a surge in China's factory output, pushing metal prices higher and boosting miners.

By 0859 GMT the FTSE 100 .FTSE was 39.43 points higher at 5,269.68, hitting a two-week high, after it closed 0.1 percent lower on Tuesday, ending a four-session winning run.

Data showing China's factory output jumped to a 19-month high in October helped sustain positive sentiment on the blue-chip index which has risen 52 percent since touching a six-year trough in March.