* Surplus power margin seen at 6.6 percent this winter
* Tightest period expected in week of Jan. 9, 2017
* Less gas storage in Rough than previous winters (Adds forecasts for LNG deliveries)
By Nina Chestney and Susanna Twidale
LONDON, Oct 14 (Reuters) - Britain's electricity supply this winter will be tight, but there will be enough power to meet demand after emergency back-up capacity was secured last year and extra capacity returned to the market this summer, National Grid said on Friday.
Britain is forecast to have a surplus power margin of 6.6 percent this winter, above the 5.5 percent forecast in July, National Grid said in its 2016/17 winter outlook report.
"We expect the electricity margin to be tight but manageable this winter," Britain's grid operator said.
National Grid said the margin figure included 3.5 gigawatts (GW) of back-up power services procured last year.
"Additional generation capacity at Eggborough power station has (also) returned to the market and an outage on the East West Interconnector has decreased exports to Ireland," the report said. "These events have had a positive impact on the outlook for the winter."
This summer Britain's Eggborough coal-fired power plant said that its unit 4 would run this winter, providing an extra 430 megawatts (MW) of capacity.
National Grid still expects to import power from France, even though power prices have recently increased there after several nuclear reactor outages.
Total winter power demand is expected to be 53.6 GW, excluding interconnector exports, National Grid said.
The system could be at its tightest in the week of Jan. 9 because of planned outages and expected colder weather driving up demand.
In Britain's gas market there will be less gas in storage than in previous winters because of technical problems at Centrica's Rough storage site.
"We expect there will be 1.3 bcm (billion cubic metres) of gas in storage at the Rough facility, compared with 2.8 bcm in October 2015 and a five-year average of 3.4 bcm," National Grid said.
Rough, Britain's largest gas storage site, has maximum capacity of 3.7 bcm.
However, there should be sufficient gas available from a range of sources, including domestic gas fields, liquefied natural gas (LNG) and imports, to meet winter demand.
National Grid gave a wide range of between 5 mcm/day and 100 mcm/day for potential winter LNG deliveries.
The amount of LNG delivered to Europe, typically from Qatar, is determned by global gas prices. In recent months higher prices in Asia have led to lower deliveries to Britain, though National Grid said that could change this winter.
Britain's total gas demand is forecast at 49.1 bcm, with peak demand seen at 472 million cubic metres per day. (Editing by David Goodman)