THIS is a general warning. Mention Joan Collins in the presence of here-there-and-everywhere Roger Thomas at your peril. Ever since St Joan stood up the Hans Andersen Club, a charity for deprived children of which Roger is president, the soap diva's name is like a red rag to a bull for the usually amiable broadcaster. ''We were disappointed when she decided not to come to Hongkong. But I've been around long enough to expect the worst when dealing with celluloid queens,'' he complained. But not to worry. Roger has enlisted the help of other caring folk like Civil Service Secretary Anson Chan and Director of Social Welfare Ian Strachan, to attend tomorrow's ball for the kids at the Furama. Some tickets are still available. For details call Ella Doswell on 338 8564.