* FTSE 100 falls 0.8 pct
* Energy stocks weigh as analysts cut earnings estimates
* Tesco up on media report of bid interest for Asian assets
By Francesco Canepa
LONDON, Oct 20 (Reuters) - Britain's top equity resumed its fall on Monday, giving back half of the gains made in the previous session as investors reckoned with reduced earnings expectations for oil companies after a slump in oil prices.
At 1044 GMT, the blue-chip FTSE 100 index was down by 53.07 points, or 0.8 percent, at 6,257.22 points, after rising 114 points on Friday.
The FTSE, which hit a 15-month low on Thursday, has been volatile in recent weeks as investors were unnerved by weak European economic data just as the U.S. Federal Reserve winds down its equity-friendly asset-purchase programme.
"There was a huge bounce up on Friday, but I don't think the volatility will go away," said Hantec Markets analyst Richard Perry.
Energy shares knocked 23 points off the FTSE as analysts slashed their profit forecasts for a number of firms in the sector in light of lower oil prices.
Brent was down 33 cents at $85.83/barrell after hitting a near four-year low at $82.60 last week, hit by weakening demand at a time of ample supply.
Goldman Sachs cut its earnings estimates for the oil services sector by between 18 percent and 22 percent, leaving the bank 20-25 percent below consensus.
"Historical correlations between oil prices and European oil service revenues imply there is a 10 percent-20 percent downside potential to revenue from 2014 (estimated) levels on scenarios of Brent oil prices at US$90/80/bl," Goldman analysts said in a note.
Oil explorer Petrofac was down 2.6 percent as Goldman, Societe Generale and Natixis cut their target prices for the company.
Gas major BG Group fell 3.1 percent, with Royal Dutch Shell down 2.3 percent.
Supermarket chain Tesco, whose shares have slumped some 20 percent over the last month after it said its profits had been overstated, beat the broader market downturn to rise 2.3 percent, making it the best-performing FTSE stock in percentage terms.
Traders attributed Monday's gain to a report in The Times that private equity companies were planning to make offers for Tesco's 9 billion pound ($14.5 billion) Asian business, and to a Sky News report that Tesco's accounting black hole would be less than originally stated.
"There's probably a bit of bargain hunting going on with Tesco at these levels, but I still think they've got more problems to face down the road," said Central Markets trading analyst Joe Neighbour. (1 US dollar = 0.6208 British pound) (Additional reporting by Sudip Kar-Gupta; Editing by Susan Fenton)