My 2008 RX400h with 135k on the clock has started to lose small amounts of coolant from a hairline crack about 100mm long I have spotted in the plastic plenum chamber (I don't know its correct term) which is the only visible part of the main coolant radiator. By small amounts I mean enough to take the header tank from max to minimum in 2000 miles - clearly however it might get worse. Lexus are quoting £800 for a replacement radiator. Does anybody have any suggestions of glues or whatever which might do the the job?