Marks & Spencer share rally lifts Britain's blue-chip FTSE index

* Benchmark FTSE 100 index rises 0.9 percent

* M&S shares jump 9.1 percent after results

* Meggitt (Other OTC: MEGGF - news) up 7.5 percent on share buyback plan (Adds detail, quotes, updates prices)

By Alistair Smout

EDINBURGH, Nov 5 (Reuters) - Britain's top share index advanced on Wednesday, as reassuring corporate reports helped counteract modest weakness in commodity-related stocks.

Retailer Marks & Spencer led the market higher, surging 9.1 percent after raising its margin outlook, while aircraft parts supplier Meggitt gained on its plan to start a share buyback programme.

Marks & Spencer (Other OTC: MAKSF - news) was the biggest gainer in the blue-chip FTSE 100 index, after Britain's top clothing retailer by revenue posted a rise in underlying first-half profit for the first time in four years and raised its guidance for non-food gross margin for 2014-15.

The update came a day after strong sales growth at discount fashion group Primark helped boost the earnings of parent AB Foods, although disappointing updates from supermarket Tesco (Xetra: 852647 - news) and fashion chain Next had fuelled concern about the retail sector.

"Earnings season has not been too bad, but retailers have been mixed. In light of what we heard from Tesco, there had been concern, but Marks and Spencers has been all right and that's come as a surprise," Mark Priest, sales trader at ETX Capital, said.

"Not all the retailers are going down the pan. People are still spending, and that's the key.

Meggitt shares rose 7.5 percent, even as the company cut its expectations for 2015 in the latest downgrade in a series, blaming delays in the U.S. military market and financial difficulties being faced by its partner in Brazil.

The FTSE 100 index was up 0.9 percent at 6,514.81 points by 1514 GMT after falling 0.5 percent in the previous session.

Chartists said that the index looked well positioned to move back towards 6,900, where the FTSE 100 had tested all-time highs before a 10 percent drop between September and October.

"It looks like the 50-day moving average (at 6,591) could well be tested very soon. If we hold above that, we could move back towards September highs," Fawad Razaqzada, technical analyst at Gain Capital (NYSE: GCAP - news) , said, adding that weakness in commodity prices could hold the index back.

The UK mining index was down 0.3 percent, underperforming all other sectors after data showing growth in China's services sector weakened further in October. China is the world's biggest metals consumer.

Precious metals miners were also hit by a fall in gold and silver prices to four-year lows, with Fresnillo (Other OTC: FNLPF - news) down 2.9 percent, the biggest decliner in the FTSE 100 index.

Traders also attributed the falls in precious metals to strength in the dollar after Republican gains in the U.S. mid-term elections.

Outside the blue chips, Stock Spirits Group Plc slumped 27.8 percent, the biggest faller on the mid-cap FTSE 250 .

The biggest vodka maker in Poland and the Czech Republic fell after it warned that the impact of higher taxes and aggressive pricing by competitors could hurt its full-year core profit. (Additional reporting by Atul Prakash; Editing by Keiron Henderson and Susan Fenton)