* Banks rally; investors cheer RBS update

* Energy stocks weak, limiting FTSE 100's gains

* BA a top blue-chip riser after Q2 results

By Tricia Wright

LONDON, Nov 6 (Reuters) - Britain's top share index rose early on Friday, as gains in banks offset a retreat by energy stocks, ahead of key U.S. monthly jobs data which could give clues as to the outlook for the world's biggest economy.

At 0909 GMT, the FTSE 100 .FTSE was up 12.28 points, or 0.2 percent, at 5,137.92.

The index closed 0.4 percent higher on Thursday as the Bank of England and the European Central Bank kept interest rates steady, while data from the U.S, which showed a rise in productivity and fall in initial weekly jobless claims, also soothed investors' nerves.

"The key figure we've got out today is the non-farm payrolls, so if it doesn't live up to expectations then I think we're going to see a big dump in these markets," said Mike Lenhoff, strategist at Brewin Dolphin.