UK GAS-Prompt prices slip on oversupply

LONDON, Sept 4 (Reuters) - Prompt British wholesale gas prices slipped on Monday morning, as strong output from the country's wind farms curbed demand from gas-fired power plants, leaving the market oversupplied. * The British within-day gas price was down by 0.45 pence at 44.85 pence/therm at 0850 GMT. * The day-ahead gas price was up by 0.15 pence at 44.85 p/therm. * Traders said expectations of high output from Britain's wind farms, coupled with weaker coal prices that boosted coal-fired power generation, had dented gas prices. * The British gas system was oversupplied by 3.9 million cubic metres (mcm), with demand forecast at 152.3 mcm and flows at 156.2 mcm/day, National Grid (LSE: NG.L - news) data showed. * Demand was 22 percent lower than the seasonal norm of 196 mcm, the data showed. * Peak wind power generation is forecast at 4.7 gigawatts (GW) on Monday, rising to 5.1 GW on Tuesday, National Grid data showed. * Coal-fired power plants provided around 6 percent of Britain's electricity, with wind around 9 percent and gas at 43 percent, National Grid data showed. * European benchmark API2 2018 coal futures were down by $0.5 per tonne at $77.60 per tonne, having traded as high as $79.50 last week. * The October gas contract was up by 0.10 pence at 44.70 p/therm. * Traders said the absence of any scheduled deliveries of liquefied natural gas (LNG) into Britain helped to lift the front-month contract. * The day-ahead gas price at the Dutch TTF hub was down by 0.03 euro at 16.45 euros per megawatt hour. * The benchmark Dec (Shanghai: 600875.SS - news) -17 EU carbon contract was up by 0.01 euro at 5.84 euros per tonne. * Thomson Reuters (Dusseldorf: TOC.DU - news) analyst view: http://emea1.apps.cp.extranet.thomsonreuters.biz/cms/?pageid=united-kingdom-gas (Reporting by Susanna Twidale, editing by Louise Heavens)