INCOME from the sale of land along the airport railway is to be put towards construction of Chek Lap Kok airport and its rail link. The income from the 62 hectares along the rail link was estimated to be $20 billion at 1991 prices, but with the booming property market, it is now thought their sale will raise at least $40 billion. A government official said the capital injection into the airport and rail projects would not be cut, despite the increase in land premiums. ''The extra land income will go to the two projects. This means that we can borrow less, resulting in a smaller interest payment,'' he said. Britain wants China to agree in one go to the sale of the 62 hectares of land, but says it will not put it all up for sale at the same time. This should be done to reassure bankers raising loans for the Mass Transit Railway Corporation - which will build the railway - and the future Airport Corporation, the official said. He expected Airport Committee land experts to discuss the issue soon. A member of the pro-China Democratic Alliance for the Betterment of Hong Kong, Ho King-on, said he believed pumping the extra land income into the two corporations could help solve the airport deadlock. Beijing had said it would grant all the land sites along the airport railway before 1997, and Mr Ho said this should allay British concerns that Chinese approval might not be forthcoming. In Sydney the Governor, Chris Patten, said the sooner the outstanding issues on the airport projects could be resolved the better it would be for Hong Kong. He welcomed the comment on Friday by the Sino-British Joint Liaison Group's Chinese senior representative, Guo Fengmin, that China would like to resume expert meetings. ''If Chinese officials believe that we should begin by a meeting of the experts' group then so be it. I think what everybody in Hong Kong would like to see is an early resolution to these problems.''