Marks & Spencer share rally boosts blue-chip FTSE index

* Benchmark FTSE 100 index rises 0.9 percent

* M&S shares gain 9.1 percent after results

* Meggitt (Other OTC: MEGGF - news) up 5.9 percent on share buyback plan (Adds quote, detail)

By Alistair Smout

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

Retailer Marks & Spencer led the market higher 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) rose 9.1 percent, 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.

"Gross margin has shown a strong improvement, in part due to the tight control of costs which M&S is currently undertaking, whilst the roll out of more 'Simply Food' stores over the next three years aims to capitalise on the way the market is developing," Richard Hunter, head of equities at Hargreaves Lansdown Stockbrokers, said.

"In addition, the group is confident that it is set for the impending Christmas period, whilst the extra cash generation has enabled a modest raising of the dividend."

Meggitt shares rose 5.9 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 6,514.66 points by 1114 GMT after falling 0.5 percent in the previous session.

However the UK mining index was flat, 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 Randgold Resources falling 2.6 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. midterm elections, which raised hopes that U.S. political deadlock would end and dented appetite for safe haven metals.

"Last night's win for the Republicans in the U.S. midterm elections has left Obama as a lame duck president and sent the greenback soaring, although in addition to driving down commodity prices it should also be good news for UK exporters," Tony Cross, market analyst at Trustnet Direct, said.

Outside the blue chips, Stock Spirits Group Plc slumped 22.7 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)