London pre-open: FTSE set to tumble at opening bell

London-listed stocks are set for a sharp slide on Friday morning, with City sources predicting the FTSE 100 will open around 75 points lower than yesterday's close of 6,431.85.

The decline comes after US markets tumbled the previous evening, as companies began reporting quarterly earnings and investors looked intent to book profits after Wednesday's gain.

"It's been an incredible week in the markets in which we've seen huge amounts of volatility that have quite frankly been driven by nothing but fear," said Alpari market analyst Craig Erlam.

"People around the world are trying to rationalise the moves and attribute them to any minor event, economic release or comment that hits the news wires, but the simple fact of the matter is that at times, there's no rationalising the irrational."

Friday's focus will shift to UK trade data, with the Office for National Statistics expected to announce the total trade balance - the difference between exports and imports of British goods and services - to be -£3,000, slightly up from July's -£3,348.

The trade balance is one of the biggest components of the UK's balance of payment and offers valuable insight into pressures on the value of the sterling, as a positive figure indicates that exports are greater than imports.

Figures for trade balance excluding the EU, an indicator of Britain's trade with countries outside of Europe, are also due to be published at 08:30, with analysts expecting a reading of -£4,000, up from -£4,345 last month.

In company news, oil and gas group Tullow Oil has reported that the Sputnik-1 well on its part-owned Arouwe block offshore Gabon has found "non-commercial" hydrocarbons. Nevertheless, Tullow's exploration director Angus McCoss said it was an "encouraging result". He said: "Having found a petroleum system, we will now take the data from this well and integrate it with our seismic data to discuss with partners our next steps for the licence."

Bus and train operator National Express announced that its finance chief was quitting to join speciality chemical group Croda International. National Express said Jez Maiden, its group finance director since 2008, had resigned to take up the equivalent role at Croda early in the New Year.

Student accommodation group Unite has acquired two new development sites, adding around 1,250 new beds to its current total. The two schemes are expected to achieve returns in line with Unite's targets for regional development of 9.5-10% yield on cost and have a combined total development cost of around £80m.