UK stocks opened flat on Tuesday morning as yesterday's optimism surrounding the key Merkel-Sarkozy meeting and an upcoming EU summit was outweighed by some worrying comments from Standard & Poor's (S&P).S&P WARNS OF MASS EUROZONE DOWNGRADEReports of a impending credit rating agency action on the Eurozone were confirmed last night as S&P announced that it placed the rating of 15 of the area's sovereigns on CreditWatch with a negative outlook.Only Greece (whose rating was already at CC categorized as "highly vulnerable, speculative bonds" with a high near-term probability of default) and Cyprus (whose rating was already on review) escaped this ratings action. S&P explained that these "CreditWatch placements are prompted by our belief thatsystemic stresses in the euro zone have risen in recent weeks to the extent that they now put downward pressure on the credit standing of the euro zone as a whole." S&P expects to complete their review "as soon as possible" following the EU summit next Thursday and Friday and concludes that "we believe that ratings could be lowered by up to one notch for Austria, Belgium, Finland, Germany, Netherlands, and Luxembourg, and by up to two notches for the other governments." WOLSELEY LEADS RISERS, MINERS MIXEDWolseley, the FTSE 100 distributor of plumbing and heating products, was the high riser early on after seeing like-for-like revenue growth of 5% in the three months to the end of October, compared to the same period of 2010. Miners were providing a drag with announcements from Xstrata and Anglo American disappointing. BHP Billiton and Kazakhmys fell into the red too.Anglo-Swiss miner Xstrata has reported "significant increases" to its total mineral resources at the Las Bambas and Corroccohuayco projects in southern Peru and at El Pachón in Argentina.Meanwhile, Anglo American said that it is to invest $1.7bn in a Queensland site in a bid to triple production of metallurgical coal from its Australian assets by 2020.AIM-listed Red Rock Resources fell 10% after warning that December gold sales will be reduced by religious holidays and scheduled maintenance work. Nevertheless, the firm assured that November sales from its Colombia project were ahead of the previous month.Fresnillo were bucking the sector trend, rising 1.3% after saying that it will pay a second interim dividend of $300m after the firm's cash pile was boosted by metal prices in 2011. Lloyds snuck into the top performers list, despite falling in the opening minutes. Banking peers HSBC and RBS however were falling.BCFTSE 100 - RisersWolseley (WOS) 1,933.00p +1.58%Fresnillo (FRES) 1,780.00p +1.31%ITV (ITV) 65.85p +0.84%ICAP (IAP) 371.30p +0.79%Rolls-Royce Group (RR.) 719.50p +0.77%Vodafone Group (VOD) 174.35p +0.75%GlaxoSmithKline (GSK) 1,410.00p +0.75%BT Group (BT.A) 190.00p +0.74%British American Tobacco (BATS) 2,954.00p +0.72%Lloyds Banking Group (LLOY) 27.18p +0.67%FTSE 100 - FallersHSBC Holdings (HSBA) 509.20p -1.70%Schroders (Non-Voting) (SDRC) 1,127.00p -1.66%Meggitt (MGGT) 379.60p -1.02%Royal Bank of Scotland Group (RBS) 22.56p -0.92%Kazakhmys (KAZ) 960.50p -0.83%BHP Billiton (BLT) 1,996.00p -0.80%Glencore International (GLEN) 421.70p -0.76%Admiral Group (ADM) 920.50p -0.75%United Utilities Group (UU.) 611.00p -0.73%British Sky Broadcasting Group (BSY) 753.00p -0.73%FTSE 250 - RisersThomas Cook Group (TCG) 16.95p +2.05%Michael Page International (MPI) 352.90p +2.02%Premier Foods (PFD) 5.70p +1.79%Personal Assets Trust (PNL) 33,910.00p +1.71%Domino Printing Sciences (DNO) 516.00p +1.47%Supergroup (SGP) 540.50p +1.41%Derwent London (DLN) 1,613.00p +1.26%Shanks Group (SKS) 109.10p +1.11%FirstGroup (FGP) 319.80p +0.98%National Express Group (NEX) 217.50p +0.79%FTSE 250 - FallersCarpetright (CPR) 442.10p -5.17%New World Resources A Shares (NWR) 452.90p -4.05%Northgate (NTG) 232.60p -3.88%Talvivaara Mining Company (TALV) 233.20p -3.76%Victrex (VCT) 1,150.00p -3.52%Stobart Group Ltd. (STOB) 112.90p -3.50%Rathbone Brothers (RAT) 1,060.00p -2.75%SDL (SDL) 645.00p -2.71%Dixons Retail (DXNS) 11.63p -2.19%Home Retail Group (HOME) 98.70p -2.18%