Marsh & McLennan Cos. Inc. has acquired 86% of Argentina's largest independent broker, Ayling Barrios S.A. The new company will be called Ayling Marsh & McLennan S.A. Terms of the transaction were not disclosed. . . .Britain's Expo rt Credit Guarantee Department is restoring export credit coverage in the next few months for British companies competing for contracts in Algeria's oil and gas sector. It withdrew the coverage in late 1991 due to Algeria's high ra te of sovereign default and its political turmoil. The ECGD said it would restore coverage for companies wanting to compete in the new round of oil and gas contracts given Algeria's "positive economic progress" and the political stability in the southern part of the country where oil and gas projects are located. . . .Royal & Sun Alliance Insurance Group P.L.C. has sold Newark, N.J.-based Newark Insurance Co., which writes personal lines business in the st ate of New Jersey, to The Robert Plan Corp. The decision to sell Newark, which in 1995 had net written premiums of $89 million, is in line with Royal & Sun Alliance's strategy of focusing on commercial lines in New Jersey. . . .U.K . broker C.E. Heath P.L.C. has opened a second office in Medellin, Colombia. It said the new office is largely to service one of its main Latin American clients, Sur-americana de Seguros. Sur-americana is one of Colombia's main ins urance companies, as well as being the holding company of a group of more than 100 companies ranging from financial institutions to textile and cement factories. . . . David Strauss was appointed chairman of Sedgwick Noble Lowndes Ltd., an employee benefits and financial services unit of Sedgwick Group P.L.C., Jan. 1. He has replaced Rob White-Cooper, who will become chief executive of Sedgwick Group April 24. Mr. Strauss will retain responsibility for SNL's European operations, which he has been running since 1994. . . . Bermuda-based Starr Excess Liability Insurance Co. Ltd. has appointed Kevin P. Gadbois as vp-underwriting in what it said is a response to continued growth of its ne w professional liability and D&O business. Mr. Gadbois was previously with the executive liability division of the Cincinnati-based Great American Insurance Co., where he was assistant vp. . . . Standard & Poor's has raised its cla ims-paying ability rating of Mercantile & General Reinsurance Co. Ltd. to AA+ from A+ and removed it from CreditWatch, where it was placed with positive implications last August. The reinsurer's U.S. non-life subsidiary, Mercantile & General Reinsurance Co. of America, remains on CreditWatch, though implications have been revised from positive to "developing"-meaning it may be either raised or lowered from its current A+ rating. . . . Chubb Corp. has incre ased to 100% its shareholding in Chubb de Mexico Compania de Seguros. Previously, Chubb had a 30% stake in the Mexico City-based multiline insurer.