Doubling the lights used to be an answer but is not any longer. Old style lights were in series and therefore 48 bulbs in a string meant they were 6v bulbs. This is no longer the case, a lot of modern lights have transformers. Christmas tree lights are really cheap now. Connecting a 110 set to 240v can be very dangerous in terms of a fire hazard, throw them in the bin.
Terry
June 2006