Signs of a pick-up in support for Scottish independence impacted on the pound and left shares in Royal Bank of Scotland sharply lower.

The movement came as a poll by YouGov showed the No camp are now six points ahead of the Yes campaign, down from 14 points in mid-August.

The FTSE 100 Index rose 3.9 points to 6829.2, with blue-chip mining stocks and a resilient performance from the major supermarkets ensuring an uncertain London market posted a third day of gains.

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Miners were pushed higher on the back of a vote by the Australian government to scrap a 30 per cent tax on mining profits.

The move saw Rio Tinto climb 5.5p to 3236.5p, while BHP Billiton was 15p higher at 1920p and Anglo American rose 38.5p to 1562p. The pound dipped against the dollar by more than 0.5 per cent to 1.65, following the referendum poll and as the greenback was beefed up by strong US manufacturing data. Sterling was also down against the euro, at below 1.26.

With Royal Bank of Scotland seen as being vulnerable to currency risks caused by a Yes vote, shares fell two per cent or 8.8p to 351.5p.

Lloyds Banking Group was down by a penny at 74.8p and Perth-based energy company SSE was 25p cheaper at 1329p.

The other main focus of the session was on the housebuilding sector after Redrow published full-year results showing a 91 per cent leap in profits.

It has benefited from the Government's Help to Buy scheme but its sales rate has slowed in the new financial year, in part due to the impact of new mortgage rules. After initially rising, shares were 2.7p lower at 278.3p, as Barratt Developments also fell 6.4p to 362.4p and Persimmon slipped 2p to 1331p.

Tesco, which warned on profits for the third time this year on Friday, was 5.4p higher at 230.9p, while Sainsbury's cheered by almost two per cent or 5.2p to 294.7p. Morrisons has borne the brunt of market fears about the sector's ongoing price wars but was 0.6p higher at 174.1p. Engineering firm Weir was the top of the FTSE 100 leaderboard after Credit Suisse upgraded its rating to outperform from neutral.

At the same time it cut its guidance on rival engineer IMI to neutral from outperform, saying Weir had a better profile and relative cash conversion. Weir lifted 67p to 2706p, IMI fell 25p to 1329p.

Pub group JD Wetherspoon lifted 12.5p to 754.5p after brokers at Numis said strong sales but weak margins will see pre-tax profits rise slightly when the firm posts full-year results next week.

The biggest risers on the FTSE 100 were Weir up 67p at 2706p, Anglo American up 38.5p at 1562p, Tesco up 5.4p at 230.9p and Hargreaves Lansdown up 22p at 1139p.

The biggest fallers were Royal Bank of Scotland down 8.8p at 351.5p, Randgold Resources down 94p 4991p, IMI down 25p at 1329p and Royal Mail down 7.8p at 440p.