Alex Garland has nothing to do with Trainspotting. It was two different writers who wrote it, one being John Hodge, who worked with Danny Boyle for 3 films and the writer of the book.
As for Porno, Danny has expressed interest in directing its sequel, but he has a handful of other projects he wishes to do first, Im not sure if he and Ewan McGregior are even talking yet and according to Botle, he is waiting for his actors to age a bit more to look more like the rough characters they will be playing who havent aged well in the 5b4ten year gap.
Hope this helps.