For several years Austravel (071-734 7755) has run cheap charter flights to Australia, and has had the market to itself. This winter, however, tour operator Airtours (0706 260000) is muscling in. The fares war between the two companies, last week saw return prices to Sydney fall as low as pounds 429. Austravel still has a fair amount of availability to Perth, Alice Springs and Sydney for pounds 499.